The Body Can Protect Itself From Infection PDF

🦠 Chapter 7 — The Body Can Protect Itself from Infection Created @March 26, 2025 12:17 PM Class Human Biology 7.1 Pathogens pathogens - disease-causing organisms are called -commun...

🦠 Chapter 7 — The Body Can Protect Itself from Infection Created @March 26, 2025 12:17 PM Class Human Biology 7.1 Pathogens pathogens - disease-causing organisms are called -communicable diseases — disease caused by foreign organisms invading the body vector — an agent capable of transferring a pathogen from one person to another Bacteria -bacteria are prokaryote — unicellular organisms with a simple internal structure — lacking a nucleus, and DNA either floats freely in cytoplasm or is in form of circular plasmids -great majority of bacteria are harmless to humans — they are non-pathogenic -huge numbers of bacteria live on skin, alimentary canal, and other parts of body -bacteria affects body differently depending on species — effects may include producing toxins or inducing allergic responses -bacteria very small, a diameter of 0.5 to 2.0 micrometers and a length from 1-10 micrometers — able to be seen under a microscope and under light, about all bacteria is seen in shape of their cells to classify them -to identify bacterium, first grown on an agar plate or growth medium in specific conditions and stained and viewed under a microscope Types of Pathogens: 2 main types — bacteria and viruses Chapter 7 — The Body Can Protect Itself from Infection 1  also fungi, parasites, and protists -bacteria are single-celled living organisms, containing DNA and RNA, having cell membranes and cell walls -reproduce rapidly by mitosis Viruses -discovered by Pasteur and Koch in 19th century, some diseases were caused by bacteria -now is known that causes of diseases are viruses -viruses range from 20-750 nanometers, too small to be seen under regular microscopes — not until 1938 when using an electron microscope to see viruses -all were found to contain genetic material in form of a molecule of DNA or RNA but never contained both -viruses are non living and non cellular — viruses rely on host cells to reproduce its genetic material 1. virus enters 2. virus RNA and RNA polymerase separate and enter the host’s nucleus 3. polymerase and activated and viral mRNA formed moves to cytoplasm 4. mRNA is used to produce viral protein and lipids. new viruses are assembled 5. viruses burst out of the cell, killing it, to infect new cells -viruses made of either DNA or RNA with a protein cover -viruses not living things as they cannot reproduce by themselves — instead they infect living cells and its DNA or RNA induces the cell to manufacture more virus particles -new virus particles are then able to leave the host cell to infect others -during process, cells become more damaged, changed, or die -viruses differ in type of cell they invade and thus symptoms shown relate to the tissue infected -some viruses multiply in bacterial cells, causing death of bacterium — such viruses known as bacteriophages Chapter 7 — The Body Can Protect Itself from Infection 2  viruses bacteria non living living need a host can live independently cell walls, cell protein coat membranes can be both DNA and only DNA OR RNA RNA usually treated with antibiotics not effective to disease antibiotics systemic localized reproduction — invades a host cell, taking control and copies RNA/ DNA destroying the host cell Transmission of Pathogens -communicable diseases are spread from person to person — some are contagious (direct transmission) while others are spread through vectors (eg; mosquitoes or flies, fleas) -modes of transmission: contact transmission — can be direct (touching an infected person) or indirect (touching contaminated objects) ingestion — consuming contaminated food or drink (eg; dysentery, salmonella) body fluid transfer — contact with infected blood or fluids can spread diseases like HIV or hepatitis droplet infection — pathogens in droplets from breathing, talking, coughing, sneezing can spread illness like Covid-19, influenza and mumps airborne transmission — evaporated moisture in exhaled droplets can be inhaled where some bacteria and viruses can be viable in particles that are lighter (eg: chicken pox and measles) transmission by vectors — transfer of pathogens by other animals such as insects, ticks, mites — some transfer pathogens directly or like house fleas Chapter 7 — The Body Can Protect Itself from Infection 3  may spread pathogen to food or water that is ingested (vector-borne diseases spread by specific vectors - malaria and dengue fever is spread by mosquitos) Direct Transmission Indirect Transmission touching vehicle borne (contaminated food, water, towels, farm tools) kissing vector borne (insects, animals) sexual intercourse airborne, long distance via dust and droplets airborne, short distance parenteral (injections with contaminated syringes) transfusion 7.2 Non-Specific Defenses Against Disease -body’s defenses classified as specific or non-specific based on what pathogens it works against -non specific defenses work against all pathogens, and are body’s first line of defense, specific defenses are directed at particular pathogens External Defenses External Defenses an effective barrier covering outside of the body and very good at stopping entry of micro-organisms, provided not broken by cuts or abrasions. at openings of skin (eyes, mouth, anus) special protection is provided by other defenses. large numbers of bacteria live on skin at all skin times, and normal bacteria occupy the area so potential pathogens find it difficult to become established. plus, skin has oily secretions of sebum produced by oil glands containing substances to kill some pathogenic bacteria. sweat secreted to skin contains salts and fatty acids to prevent growth of many micro organisms mucous membranes line body cavities opening to the exterior, which secrete mucus to trap particles and inhibit their entry to the organs of mucus body — the digestive, urinary, and reproductive tracts all protected by these membranes hair found in nasal cavity of nose, and ears — in nose, layers of mucus hairs trap up to 90% of particles inhaled Chapter 7 — The Body Can Protect Itself from Infection 4  cilia are tiny hair-like projections from cells capable of beating motions — mucous membranes lining nasal cavity and other air passages have cilia. cilia beating of cilia moves mucus containing trapped particles towards the throat where it may be coughed up or swallowed stomach juices are strongly acidic and kills many of bacteria taken in with food or contained within mucus from nose or windpipe — vagina also has acids acid secretions that reduce growth of micro-organisms. urine and sweat on skin also slightly acidic lysozyme is an enzyme that kills bacteria — eyes protected by flushing lysozyme action of tears containing the enzyme, and also found in saliva, sweat, secretions of nose and tissue fluid cerumen or ear wax, protects outer ear against infection by some cerumen bacteria, plus is slightly acidic and contains lysozyme flushing action of body fluids helps keep some areas free of pathogens — urine flowing through urethra has a cleansing effect and prevents movement of bacterial growth to stop bacteria reaching bladder and kidneys. women fluid have a shorter urethra than men and tend to suffer more with bladder infections. tears, sweat and saliva involved with flushing and cleansing Protective Reflexes -reflex is an automatic, involuntary response to a stimulus to protect the body from injury (a blink reflex, or from infection like vomiting) 1. Sneezing — stimulus for sneezing is irritation of walls of nasal cavity. irritation can be caused by noxious fumes / dust particles, likely carrying micro- organisms. forceful expulsion of air from the lungs carries mucus, forcing particles and irritating gases out through the nose and mouth 2. coughing — stimulus is irritation in the lower respiratory tract (bronchi and bronchioles). similarly to sneezing, air forced from lungs to try exhale irritant, and the air dries mucus and foreign matter up the trachea to mouth and throat 3. vomiting — psychological stimuli, excessive stretching of the stomach and bacterial toxins can induce vomiting. contraction of abdomen muscles and diaphragm, not the contraction of the stomach, expels stomach contents Chapter 7 — The Body Can Protect Itself from Infection 5  4. diarrhea — irritation of small and large intestines by bacteria, viruses or protozoans. irritation causes increased contractions of muscles and walls of intestines so irritant is removed as quickly as possible and inadequate water absorption in large intestine occurs, making feces watery Internal Non-Specific Defenses Phagocytosis -organisms that penetrate external defenses are attacked by phagocytes phagocytes are specialized white blood cells, or leucocytes that engulf and digest micro-organisms and cell debris -this eliminates many pathogens before an infection has a chance to take hold -a number of different types of cells that are phagocytic 1. Monocytes and macrophages — when tissue becomes infected or inflamed, monocytes leave the bloodstream and enter tissue. in tissue, they differentiate into macrophages (large phagocytic cells). some macrophages move through tissues looking for and destroying pathogens, others often are fixed in once place and deal with pathogens that come to them. macrophages are particularly important in removing microbes and dying cells through phagocytosis 2. neutrophils — neutrophils are described as a granulated leucocyte, due to granules visible in their cytoplasm — also characterized by their lobulated nucleus. -neutrophils are the most abundant leucocyte, accounting for 55-70% of all. during infection, neutrophils are first cells to move into tissue to destroy the pathogen by phagocytosis and are important in killing pathogens inside cells. -neutrophils have short life spans and die after a few days and dead cells make up a large portion of the pus that forms an infection 3. dendritic cells — dendritic cells characterized by projections from the cytoplasm and slightly different from macrophages and neutrophils in their function goes beyond just phagocytosis Chapter 7 — The Body Can Protect Itself from Infection 6  -cells have ability to detect, engulf and process foreign particles, and then use information about the ingested particles to assist with specific immunity Inflammatory Response -words ending with ‘’itis ‘’ — means inflammation of specific organs or tissues (tonsillitis, meningitis) -inflammation is a response to damaged tissue and the purpose is to: reduce spread of other pathogens, to destroy them and prevent entry of additional pathogens remove damaged tissue and cell debris begin repair of damaged tissue -4 signs of inflammation — redness, swelling, heat, and pain -damage to tissues stimulate a series of steps in inflammatory response — some steps assisted by proteins in the complement system produced by liver cells and macrophages -complement system is series of more than 20 proteins, many of which normally inactive and when initiated, one protein activates the next like dominoes 1. mechanical damage or local chemical changes causes specialized leucocytes called mast cells to be activated by complement proteins — results in release of histamines, heparin, and other chemicals into tissue fluid 2. histamine increases blood flow through the area due to vasodilationo to make walls of blood capillaries more permeable. more fluid moves through capillary walls into tissue and thus increased blood flow causes heat and redness, and escape of fluid from blood causes swelling 3. heparin prevents clotting, so release of heparin from mast cells prevents clotting in immediate injury area — a clot of fluid forms around the damaged area, slowing the spread of pathogen into healthy tissue Chapter 7 — The Body Can Protect Itself from Infection 7  4. complement system proteins and some chemicals are released by mast cells attract phagocytes, particularly neutrophils to actively consume micro- organisms and debris by phagocytosis 5. abnormal conditions in tissue stimulate pain receptors, and person feels pain in inflamed areas 6. phagocytes, filled with bacteria, debris and dead cells, begin to die — dead phagocytes and tissue fluid form a yellow liquid of pus 7. new cells are formed by mitosis, and repair of damaged tissue takes place Fever fever — an increased in body temperature due to hypothalamus resetting body’s thermostat to a higher level -triggered by pyrogens, chemicals released during an infection — a key pyrogen is interleukin-1 produced by activated macrophages or dendritic cells -the body responds by vasoconstriction and shivering to conserve the heat and raise temperature -when fever breaks (crisis point), the hypothalamus resets to normal, causing vasodilation and sweating to cool down Fever helps fight infection by: inhibiting bacterial growth and viral growth increasing chemical reaction rates in aid to repair tissue enhance the function of interferons — proteins that help combat viruses -though, excessively high fevers (above 44 degrees Celsius) can cause convulsions or death Lymphatic System lymphatic system consists of: a network of lymph capillaries joined to larger lymph vessels Chapter 7 — The Body Can Protect Itself from Infection 8  lymph nodes, located along length of some lymph vessels -main function of lymphatic system to collect the fluid that escapes from blood capillaries and return it back to the circulatory system -in addition, lymphatic system important in internal defense against pathogenic organisms -lymph nodes at intervals along lymph vessels, where each node contains masses of lymphoid tissue, cells of which are crisscrossed by a network of fibers -lymph entering lymph nodes contains cell debris, foreign particles and micro- organisms that have penetrated the body’s externa; defenses -some may be pathogenic, and must be destroyed to not cause disease, and some larger particles, such as bacteria, are trapped in the meshwork of fibers as lymph flows through spaces of nodes — macrophages ingest and destroy these particles by phagocytosis -when infection occurs, formation of lymphocytes increases, and lymph nodes become swollen and sore (eg: an infected finger may result in swelling / tenderness of armpit where a large number of lymph nodes lie_ -most lymphocytes important in specific immune responses to particular pathogens Specific Defenses Against Disease -body makes antibodies to combat viruses — antibodies only effective against certain viruses and are specific to immune system -immune system composed of cells and proteins to protect against foreign organisms, chemicals and cancerous or abnormal cells -some cells non-specific like phagocytes (readily able to engulf and digest pathogens) or others like B-cells and T-cells only provide protection against specific micro-organisms causing substance — when cells react, it is called the immune response Chapter 7 — The Body Can Protect Itself from Infection 9  Immune Response -immune response is a homeostatic mechanism to help deal with invasion of micro organism and restore internal environment -key cells involved are B-cells and T-cells which are white blood cells called lymphocytes — mature by different routes -B and T-cells both produced in bone marrow and end up in lymphoid tissue but mature by different routes -about half of cells produced in marrow go to the thymus, where they mature into T-cells before being incorporated in lymphoid tissues, and other half mature in bone marrow to become B-cells and then become part of lymphoid -most of lymphoid tissue is in lymph nodes but also occurs in parts of body — spleen, thymus, tonsils Two parts of Immune Response humoral response or antibody-mediated response — involves production of special proteins called antibodies by B-cells, which circulate around the body and attack invading agents cell-mediating response by T-cells, involving formation of special lymphocytes to destroy invading agents Antigens -antibody-mediated and cell-mediated both trigged by antigens antigen — any substance capable of causing a specific immune response -they are large molecules like proteins, carbs, lipids or nucleic acids and may include: virus particles whole micro-organisms part of a bacterium toxins Chapter 7 — The Body Can Protect Itself from Infection 10  pollen grains egg whites -large molecules produced in a persons body do no cause an immune response and called self-antigens -foreign compounds that do not trigger an immune response are non-self antigens -immune system becomes programmed before birth to distinguish between self and non self antigens Antibodies -an antibody is a Y-shaped specialized protein produced by plasma cells in response to a non-self antigen -antibodies belong to a protein group called immunoglobulins (denoted as Ig) -there are five classes of antibodies which vary in structure and designated IgA, IgD, IgE, IgG, and IgM -antibody produced in response to an antigen can combine with that antigen to form an antigen-antibody complex -antigen molecules have specific active sites with a particular shape — the antibody has the complementary shape, allowing the two molecules to fit together like a key and lock -each antibody can combine with only one particular antigen in the same way as a lock and key Antigen-Presenting Cells -when a non-self antigen enters a body, specific cells recognize this to respond appropriately -these cells called antigen-presenting cells include dendritic cells, macrophages and undifferentiated B cells These cells: detect presence of a non-self antigen Chapter 7 — The Body Can Protect Itself from Infection 11  engulf the pathogen digest the pathogen and produce small fragments that move to the surface of cell present the antigen to lymphocytes Antibody-Mediated Immunity -humoral response involves production and release of antibodies into blood and lymph — this is antibody-mediated immunity that provides resistance to viruses, bacteria, and bacterial toxins before micro-organisms enter body’s cells -lymphoid tissue contains thousands of types of B-cells -B-cells recognize and respond to specific antigens -antigen-presenting cells (APC’S) present antigens to B-cells and activate them -Helper T-cells also recognize the antigen and release cytokines to activate B- cells -activated B cells divide and form a clone most become plasma cells which secrete antibodies some become memory cells for faster future responses -primary response (first exposure to an antigen) a slow response (several days) B-cells multiply, differentiate into plasma cells, and produce antibodies antibody levels peak, then decline leaves memory cells for faster future response -secondary response (subsequent exposure to the same antigen) memory cells quickly recognize the antigen plasma cells rapidly produce higher antibody levels faster response, often preventing illness How Antibodies Work Chapter 7 — The Body Can Protect Itself from Infection 12  -different antibodies protect the body by different methods They may: inactivate foreign enzymes or toxins by combing them with or inhibiting their reaction with other cells or compounds bind to the surface area of viruses and prevent them entering cells coats bacteria so that they are more easily consumed by phagocytes causes particles like bacteria, viruses, or foreign blood cells to clump together known as agglutination dissolve organisms react with soluble substances to make them insoluble and thus more easily consumed by phagocytes Chapter 7 — The Body Can Protect Itself from Infection 13  Cell-Mediated Immunity -cell-mediated immunity provides resistance to intracellular phase of bacteria and viral infections — pathogens like bacteria responsible for tuberculosis and Legionnaires disease, specialize in invading and replicating inside their hosts own cells and make it difficult to overcome -cell mediated immunity also is important fighting ‘whole cells’ such as providing resistance to fungi and parasites and reject foreign tissue transplants — also to fight cancer cells -T-lymphocytes are responsible for cellular immunity — they occur in same lymphoid tissue as B-cells but occupy different areas of tissue -like B-cells, there’s thousands of types of T-cells, each type responding to only one antigen -when a foreign antigen like a virus enters the body, the antigen-presenting cell presents the antigen to the particular type of T-cells and these become activated or sensitized -sensitized T cells enlarge and divide to each give rise to a clone, a group of identical T cells — some cells of clone remain in lymphoid tissue as memory cells, able to quickly recognize the original invading antigen -if infection of same antigen occurred again, these cells can initiate much faster response to the second and subsequent infections. The cells that do not become memory cells develop further producing 3 different T-cell types: 1. Killer T-cells (cytotoxic T-cells) — migrate to site of infection and deal with invading antigen, attaching to the invading cell and secrete a chemical to destroy the antigen, and then, go search of more antigens 2. Helper T-cells — plays important role in both humoral / cellular immunity. they bind to the antigen on antigen-presenting cells, stimulating secretion of cytokines that: attract lymphocytes to infection site which become sensitized an activated, thus intensifying the response Chapter 7 — The Body Can Protect Itself from Infection 14  attract macrophages to the place of infection so macrophages can destroy the antigens by phagocytosis intensify the phagocytic activity of macrophages promote action of kill T-cells 3. Suppressor T-cells — act when immune activity becomes excessive or infection has been successfully dealt — they release substances that stop T and B cells, slowing down immune response Types of Immunity -immunity — resistance to infection by invading micro-organisms -presence of memory cells allow body to respond quickly enough to deal with invasion of any pathogenic micro-organism before symptoms of disease occur -ability to respond rapidly may be natural or artificial -natural immunity occurs without human intervention, artificial immunity results from being given an antibody or antigen -natural / artificial immunity can be passive or active Passive Immunity -when a person receives antibodies produced by someone else — meaning individuals body plays no part in production of antibodies -can occur naturally when antibodies from mother passed across the placenta to a developing fetus or when mothers antibodies passed through breast milk -can be gained artificially when person is injected with antibodies to combat a particular infection — often done when person exposed to serious disease- causing pathogens like tetanus, rabies -antibodies given so immunity is established immediately -passive immunity is short-lived, lasting only until antibodies are broken down and excreted Chapter 7 — The Body Can Protect Itself from Infection 15  Active Immunity -active immunity when body is exposed to a foreign antigen and manufactures antibodies in response to the antigen -while amount of antibody decreases, this type of immunity lasts longer than passive immunity due to presence of memory cells -should a subsequent infection involving the same antigen occurs, the appropriate antibodies can be produced very quickly and eliminates the antigen before infection can produce symptoms -active immunity to a disease can develop from having a disease and recovering (natural active immunity) or from an injection of antigens associated with the disease (artificial active immunity) 7.4 Prevention and Treatment of Disease Vaccines -immunization means programming immune system so the body can respond rapidly to infecting micro-organisms — or developing an immunity -can occur naturally or artificially — vaccination is artificial introduction of antigens of pathogenic organisms so the ability to produce appropriate antibodies is acquired without person having to suffer the disease -a slight difference between vaccination and immunization a vaccine is the antigen preparation used in artificial immunization Traditional vaccines are of 4 types: live attenuated vaccines: living attenuated micro-organisms are those with reduced virulence — micro-organisms with reduced ability to produce disease symptoms. thus, immunized person does not contract disease but manufactures antibodies against the antigen -vaccines containing living attenuate micro-organisms including those for immunization against polio, tuberculosis, rubella, measles, mumps, and yellow fever Chapter 7 — The Body Can Protect Itself from Infection 16  inactivated vaccines: contains dead micro organisms (eg: cholera, typhoid, whooping cough) — immunity is shorter-lasting than live vaccines toxoid vaccines: uses inactivated bacterial toxins (toxoids) instead of bacteria (eg: diphtheria, tetanus) sub-unit vaccines: uses fragments of microorganisms instead of the whole pathogen (HPV, hepatitis B) -an approach to modifying characteristics of pathogen by slightly changing the DNA in the micro-organisms cell — making the pathogen less virulent -another method is to insert certain DNA sequences from pathogen into harmless bacterial cells — chosen DNA sequence causes the production of antigens that are characteristics of the pathogen -vaccination with harmless bacterium results in immunity against the pathogen -likely that many future vaccines will be made using recombinant DNA Vaccine Delivery -most common to inject vaccine from syringe -another method is polio vaccine given by mouth in a sweet syrup / lumps of sugar (method no longer used in Australia but in other countries) -other forms under research, including fine spray, skin patches, ingestion of food Vaccination Schedule -vaccinations typically start at 2 years old; most diseases require multiple doses -newborns have antibodies from the mother, which can interfere with early vaccinations -the hepatitis B vaccine is an exception and given at birth due to infection risk during delivery -a single vaccine dose not usually enough — antibody levels decline, requiring a booster shot -booster shot triggers memory cells. producing stronger and longer-lasting immunity Chapter 7 — The Body Can Protect Itself from Infection 17  -must be a gap (2 months) between doses to prevent maternal antibodies from neutralizing vaccine before immune activation -other vaccinations such as cholera, yellow fever, and typhoid may be recommended depending on destination Vaccination of Populations -vaccination programs not only reduce chance of disease in the most susceptible individuals but also increase immunity of the population -such immunity known as herd immunity and depends on a high proportion of individuals being immunized Factors to consider: vaccination benefits — vaccines protect individuals and comumunities, lower health care costs, may be required for school enrolement vaccine safety — vaccines undergo strict testing and regulation before approval for effectiveness and safety misinformation — internet and media contribute to misinformation about vaccines, so important to verify Factors Affecting Decisions: Health: allergic reactions: some people may react to components used in vaccine production such as egg proteins or yeast preservatives: vaccines contain preservatives, no health risks been proven Social Factors: use of animals: some vaccines require animal tissue for production (influenza viruses grown in fertilized eggs) use of human tissue: some vaccines developed using human tissue, raising ethical concerns Chapter 7 — The Body Can Protect Itself from Infection 18  concerns of sexual activity of teenagers: some believe vaccinating against human papilloma virus encourages teenagers to be sexually active Economic: cost of vaccine — vaccines may be expensive for people to afford commercialization: interests of commercial vaccine production may affect use Antibiotics antibiotics — drugs used to fight infections of micro-organisms, particularly bacteria -cannot be used to treat viral infections — each antibiotic effect for only certain types of bacterial infection, testing often carried out prior to antibiotics prescribed -first antibiotic identified is penicillin, discovered the mold penicillin notatum able to stop growth of staphylococcus -penicillin works by preventing synthesis of walls of bacterial cells, inhibiting the reproduction of bacteria — about 30% of antibiotics used in Australia are penicillin based -effectiveness reduced because of bacteria resistance development -new antibiotics developed: streptomycin, erythromycin, neomycin, tetracycline, vancomycin, which interfere with protein synthesis in the cells of the target bacteria cephalosporin, which interferes with synthesis of cell wall — much likely than penicillin to produce allergic reactions -there are 2 types of antibiotics: bactericidal antibiotics — kill bacteria by changing structure of cell wall or cell membrane, or by disrupting action of essential enzymes bacteriostatic antibiotics — stops bacteria from reproducing, usually by disrupting protein synthesis bactericidal antibiotics bacteriostatic antibiotics Chapter 7 — The Body Can Protect Itself from Infection 19  kill bacteria by changing structure of cell wall or cell stops bacteria from reproducing, membrane, or by disrupting action of essential usually by disrupting protein enzymes synthesis -both types are effective in treating bacterial infections -some antibiotics affect wide range of different bacteria types — called broad- spectrum antibiotics -others, narrow-spectrum antibiotics — effective in fighting against specific bacteria -widespread use of antibiotics has created a major problem — some bacteria killed by antibiotics has gradually evolved and become resistant to them -in the past, a different type of antibiotic was the solution, but now most strains are resistant to most or all types -total drug resistance — totally resistant strains Antivirals -used specifically for treating viral infections — as antibiotics ineffective against viruses, no treatment for common cold, chickenpox, and measles -led to a hunt for chemicals used as antivirals -viruses enter a host cell, and virus of DNA or RNA induces the cell to produce new virus particles, which particles leave the cell and infect new host cells -way in which viruses replicate makes it difficult to find new drugs that will treat viral infections as host cell produces the new virus particles, any drug that interferes with viral replication likely to be toxic to the host -unlike most of antibiotics that destroy pathogenic bacteria, antivirals inhibit the development of the virus -most antiviral drugs now available target HIV, herpes, hepatitis B and C, and influenza A or B Chapter 7 — The Body Can Protect Itself from Infection 20