Chain of Infection

• The immune system protects the body from external invaders and internal changes like cancer.

Chain of Infection

• Infectious illnesses are passed from one person to another via a series of steps known as this.
• New infections can be avoided by interfering with any of these steps.
• The chain of infection has six major links:
  • Pathogen
  • Reservoir
  • Portal of exit
  • Means of transmission
  • Portal of entry
  • New host.

Pathogen

• The infectious disease cycle starts with this, a microorganism that causes disease.
• Examples include HIV, SARS-CoV-2, and the tuberculosis bacterium.
• Illness is frequently caused by pathogens that either produce toxins that harm human tissue or directly invade body cells.

Reservoir

• It has a natural habitat where it normally exists.
• A person, animal, or environmental component such as soil or water can serve as this.
• A human who is a reservoir for a pathogen may be sick or an asymptomatic carrier.
• Asymptomatic carriers spread infection without having any symptoms.

Portal of Exit

• The pathogen needs to leave the reservoir.
• Portals of exit include:
  • Saliva (e.g., mumps)
  • Mucous membranes (e.g., STIs)
  • Blood (e.g., HIV, hepatitis)
  • Feces (e.g., intestinal infections)
  • Nose and throat discharges (e.g., colds, influenza).

Means of Transmission

• Transmission can be direct or indirect.
• Direct transmission: Pathogen is passed without an intermediary.
• Requires close association but not necessarily physical contact.
• Sneezing and coughing can discharge pathogens into the air.
• Proper handwashing is key to preventing direct transmission of respiratory and intestinal infections.
• Sexual contact and contact with blood are other means of direct transmission.
• Indirect transmission: Animals and insects (vectors) carry the pathogen.
• Pathogens can also be transmitted via contaminated soil, food, water, or inanimate objects (e.g., utensils, doorknobs).
• Some pathogens float in the air on dust particles or droplets.

Portal of Entry

• A pathogen needs this , into the body to infect a new host.
• Entry can occur in three ways:
  • Direct contact/penetration of the skin
  • Inhalation
  • Ingestion
• Pathogens entering the skin or mucous membranes can cause local or systemic infections.
• STIs enter through the mucous membranes of the urethra or cervix.
• Respiratory secretions can cause upper respiratory infections, pneumonia, or systemic infections.
• Food-borne and water-borne organisms enter the mouth and attack the cells of the small intestine or colon, causing diarrhea.
• They also enter the circulation through the digestive system and spread to other regions of the body.

The New Host

• A strong immune system or resistance to a particular pathogen is crucial.
• A greater number of pathogens entering can increase the chance of overwhelming the body's defenses.
• If conditions are right, the pathogen will multiply and produce disease.
• The new host can then become a reservoir, starting a new chain of infection.

Breaking the Chain

• Interfering with the chain of infection at any point can prevent disease.
• Strategies include public health measures and individual actions.
• Reservoirs can be isolated via quarantine, or insects/animals carrying pathogens can be killed.
• Public sanitation practices can kill pathogens.
• Transmission can be disrupted through handwashing and the use of face masks.
• Immunization and treatment can stop pathogens from multiplying and spreading.

Physical and Chemical Barriers

• The body’s first line of defense is physical and chemical barriers.
• The skin prevents many microorganisms from entering.
• Mucous membranes in the mouth contain cells that prevent the passage of unwanted organisms and particles.
• Body openings and fluids (tears, saliva, vaginal secretions) contain antibodies and enzymes that destroy microorganisms.
• The respiratory tract is lined with mucous membranes and cilia, which sweep foreign matter out.
• Damaged ciliated cells result in smokers developing a chronic cough to compensate for airway damage.

The Immune System

• It operates through a network of cellular defenders.
• It is activated once the body has been invaded by a foreign organism, such as inflammation and immune responses.

Immunological Defenders

• The immune response is carried out by different types of white blood cells produced in the bone marrow.
• Neutrophils attack and ingest pathogens in the bloodstream.
• Macrophages take up stations in tissues and devour pathogens and worn-out cells.
• Natural killer cells directly destroy virus-infected and cancerous cells.
• Dendritic cells eat pathogens and activate lymphocytes.
• Lymphocytes (T cells and B cells) travel in the bloodstream and lymphatic system.
• Lymph nodes filter bacteria and substances from the lymph.
• Swollen lymph nodes indicate an active immune response.
• T cells differentiate into helper, killer, and suppressor T cells.
• B cells produce antibodies.
• Memory T and B cells enable a rapid response to future invasions.
• The immune system distinguishes foreign cells from the body’s own cells via markers on the cell surfaces.
• Lymphocytes that attack the body cause autoimmune diseases.
• Non-self markers that trigger the immune responses are antigens.
• Antibodies have complementary surface markers that lock onto antigens, triggering the destruction of the invading pathogen.
• The body has antibodies for potentially billions of antigens.

The Inflammatory Response

• It occurs when the body is injured or infected.
• Special cells release histamine, causing blood vessels to dilate and fluid to flow into the injured tissue.
• This results in increased redness, heat, and swelling.
• White blood cells attack the invaders.
• Pus, a collection of dead white blood cells and debris, may form at the site of infection.

The Immune Response

• The immune system responds in two ways: natural (innate) and acquired(adaptive).

• Natural response:

  • Neutrophils, macrophages, dendritic cells, and natural killer cells
  • They recognize pathogens as “foreign” but have no memory of past infections.
  • They eat the invaders or destroy infected body cells.

• Acquired response:

  • T and B cells
  • They change after contact with the pathogen, developing memory for the antigen.

• The immune response happens in four main phases.

• Phase 1: Dendritic cells consume foreign cells and display their antigens to helper T cells.

  • Also known as the detection phase.

• Phase 2: Helper T cells multiply rapidly, triggering the production of killer T cells and B cells in the spleen and lymph nodes.

  • Also known as amplification phase.

• Cytokines regulate and coordinate the immune response, stimulating production of immune cells and promoting their activities.

• Phase 3:

  • Killer T cells attack infected cells by recognizing antigens displayed on their surfaces.
  • Cell-mediated immune response is when the attack is carried out by cells.
  • B cells produce large quantities of antibodies that bind to antigen-bearing targets and mark them for destruction by macrophages.
  • Antibody-mediated immune response is when this type of response occurs
  • Antibodies work against bacteria, viruses, and other substances outside cells, but not infected cells.
  • The Phase 3 is known as the attack phase and can be either cell-mediated or antibody-mediated.

• Phase 4: The immune response slows down as regulatory molecules and cells inhibit lymphocyte proliferation and induce lymphocyte death.

• Debris is scavenged by white blood cells, filtered by the liver, spleen, and kidneys, and excreted from the body.

  • Known as the slowdown and cleanup phase.

Immunity

• Survival after an infection often confers immunity.
• Memory T and B cells recognize and destroy the antigen before it can cause illness.
• This subsequent response takes only a few days to initiate and prevents illness.
• This is the ability of memory lymphocytes to remember previous infections.
• This is due to memory T and B cells and a fast secondary response.

Symptoms and Contagion

• Incubation is when viruses or bacteria multiply in the body silently
• Contagion: at the time when you are yet to show symptoms of the illness.
• Prodromal period: Symptoms first appear after incubation.
• If the infected host has acquired immunity, the infection may be eradicated during the incubation or prodromal period.
• Symptoms of an illness are often due to the immune response rather than the pathogen itself.
• Fever is caused by the release of cytokines, which reset the body's thermostat.
• A runny nose occurs when lymphocytes destroy infected mucosal cells, leading to increased mucus production.
• Malaise and fatigue are caused by pro-inflammatory cytokines.
• Symptoms are immune-related and can include fever and fatigue.

Immunization

• It is based on the immune system's ability to remember previously encountered organisms.
• The immune system is primed with an antigen similar to the pathogenic organism but not as dangerous.
• The body responds by producing antibodies, preventing serious infection upon exposure to the disease organism itself.
• This is due to the principles of immunization.

Types of Vaccines

• Live, attenuated organisms: Weakened microbes are used in vaccines.
• Killed viruses: Pathogens are killed but retain their ability to stimulate antibody production.
• Portion of killed virus: Enough for the body to recognize the virus and make antibodies.
• Viral-vector: Another harmless virus delivers a small portion of the virus to become immune from.
• mRNA: Messenger RNA is delivered to the patient, causing cells to produce a tiny portion of the virus for the immune system to recognize.
• Active immunity: The vaccinated person produces their own antibodies.
• Passive immunity: Injection of antibodies produced by other humans or animals.
• Gamma globulin: Provides rapid but temporary immunity.

Vaccine Safety

• Side effects from immunization are usually mild.
• The risk from vaccines must be balanced against the risk posed by the diseases they prevent.
• The Public Health Agency of Canada monitors reports of adverse reactions and develops safer vaccines.
• A 2004 Institute of Medicine report found no evidence linking immunization and autism.
• Anti-vaccine conspiracy theories have detrimental effects on vaccination intentions.
• COVID-19 vaccines have been closely scrutinized and proven statistically safe and effective.
• The section includes a discussion of the risks versus benefits, monitoring, and debunking myths.

Allergy

• Results from a hypersensitive and overactive immune system.
• The immune system responds to a harmless substance (allergen).
• Allergy symptoms result from the immune response rather than the allergen.
• This section defines what an allergy is.

Allergens

• provoke a response if inhaled, swallowed, or in contact with the skin.
• Common allergens include:
  • Pollen
  • Animal dander
  • Dust mites
  • Cockroaches
  • Moulds and mildew
  • Foods
  • Insect stings
  • Medications
  • Plants (e.g., poison oak)
  • Latex
  • Metals (e.g., nickel)
  • Cosmetics

The Allergic Response

• allergic reactions is caused by the production of immunoglobulin E (IgE).
• Initial exposure sensitizes the immune system, causing production of allergen-specific IgE, which binds to mast cells.
• Subsequent exposure causes the allergen to bind to IgE, causing mast cells to release histamine and other compounds.
• Histamine causes increased inflammation and mucus production.
• Symptoms depend on the affected area:
  • Nose: Congestion, sneezing
  • Eyes: Itchiness, tearing
  • Skin: Redness, swelling, itching
  • Intestines: Bloating, cramping
  • Lungs: Coughing, wheezing, shortness of breath
• The allergic response involves IgE and histamine.

Asthma

• Is made of spasm of the muscles surrounding the airways and caused by inflammation of the airways
• The spasm causes constriction
• inflammation is the reason for obstructing the passages
• Symptoms: tightness in the chest, wheezing, shortness of breath
• Triggers: dust mites, mould, animal dander, pollen
• Treatment: muscle-relaxing medication (bronchodilator), anti-inflammatory drug
• Prevention: Testing peak airflow, avoid allergens when possible

Anaphylaxis

• Is a rare, but a most serious allergic reaction
• Results from histamine release throughout the body.
• Requires immediate treatment with epinephrine.

Dealing with Allergies

• Requires immediate epinephrine injection.
• Avoidance: Minimize exposure to allergens.
• Medication: Antihistamines, corticosteroids.
• Immunotherapy: Allergy shots desensitize a person to a particular allergen.

Pathogens and Disease

• Infectious diseases cause more than 25 percent of deaths worldwide each year.

• Pathogens include:

  • Bacteria
  • Viruses
  • Fungi
  • Protozoa
  • Parasitic worms
  • Prions

• Infections can occur almost anywhere in the body.

Bacteria

• Are single-celled microorganisms that reproduce by splitting in two.
• Classified by shape: bacilli, cocci, spirochete, or vibrios.
• The human body harbours both helpful and harmful bacteria.
• Pathogenic bacteria can disrupt the normal harmony in the intestines or cause infection in the bloodstream, tissues, and organs.

Pneumonia

• Is the Inflammation of the lungs caused by:
  • infection with bacteria
  • infection with viruses
  • infection with fungi OR by
  • contact with chemical toxins
  • contact with irritants
• Can be serious if alveoli become clogged with fluid, preventing oxygen from reaching the bloodstream.
• Bacterial pneumonia can be treated with antibiotics.
• A vaccine is available for pneumococcus bacteria.

Meningitis

• Is the Infection of the meninges (membranes covering the brain and spinal cord).
• Viral meningitis is mild and goes away.
• Bacterial meningitis is life-threatening and requires antibiotics immediately
• Routine vaccination of children reduced the occurrence of Hib meningitis.
• Symptoms:
  • fever
  • severe headache
  • stiff neck
  • sensitivity to light
  • confusion
• Can be fatal or cause permanent hearing loss.

Strep Throat and Other Streptococcal Infections

• Spherical bacterium that grows in chains.
• Characterized by: a sore throat that is red, fever, swollen lymph nodes ,and white patches on the tonsils
• Spread through respiratory droplets.
• If strep is not treated can develop into rheumatic fever
• Necrotizing fasciitis: Can also cause a deeper layer skin infection that is serious
• Other species include pneumonia, serious infections in pregnant women and newborns, endocarditis

Toxic Shock Syndrome and Other Staphylococcal Infections

• Is a deadly toxin produced by the bacteria which causes high fever, shock, peeling skin rash and inflammation Is first diagnosed in the the case where women were using highly absorbent tampons (where the bacteria allowed growth) Staphylococcus aureus is also responsible for many and also causes bacterial skin infections MRSA: (methicillin-resistant Staphylococcus aureus) causes pain lesions and has become one of the most common causes of skin infections
• It is antibiotic resident

Tuberculosis (TB)

• Chronic bacterial infection, usually affecting the lungs.
• Spread via the respiratory route.
• Symptoms: Coughing, fatigue, night sweats, weight loss and fever.

Tick-Borne Infections

• Deer tick transmits Lyme disease bacteria.
• Lyme disease symptoms occur in three stages that may include: -bull’s-eye-shaped rash
  • impaired coordination
  • partial facial paralysis
  • heart rhythm abnormalities
  • chronic/recurring arthritis.
• Rocky Mountain spotted fever and typhus caused by rickettsias bacterium.
• Lyme disease is tick-borne.

Ulcers

• Sore in the stomach lining or small intestine
• Helicobacter pylori causes up to 90% of ulcers
• Symptoms:
  • gnawing
  • burning pain in the abdomen
  • nausea
  • loss of appetite.
• Antibiotics cure the infections and ulcers.
• H. pylori causes ulcers.

Tetanus

• Caused by the bacterium Clostridium tetani which produces a deadly toxin
• Toxin leads to spasms and stiffness of muscles
• Rare in Canada due to vaccination

UTI

• Infection of the urethra and bladder
• Most times, the bacterium Escherichi coli can cause infections
• Those who are sexually active are likely to have infection of the bladder
• Antibiotics are used to treat the infection.

Actions of Antibiotics

• Antibiotics interrupt the production of bacteria
• Penicillin inhibits the formation of the cell wall when bacteria divide
• Others interfere directly with the of genetic material (DNA) during bacterial reproduction.

Antibiotic Resistance

• Develops when antibiotics are misused or overused.
• Bacteria can become resistant through genetic mutation or transfer of genetic material.
• Resistant bacteria can grow and flourish, while antibiotic-sensitive bacteria die off.
• Promoted by failing to take the full course of an antibiotic or taking antibiotics for viral infections.

Viruses

• Viruses lack all the enzymes essential to energy production and normal protein synthesis
• Viruses are parasites
• They will take whatever they need from the cells that they invade
• Infections are often caused by viruses
• Viruses are not usually treatable with antibiotics like bacteria are.

The Common Cold

• Typically contracted by hand or objects that are touched by people around you.
• Exposure to cold temperatures is not caused by exposure to hot or cold
• Often more colds occur during winter and fall cause more time is often spent indoors
• Usually a virus
• Can cause serious and further affect ones health

Influenza

• Infection caused by the virus influenza, of the respiratory tract
• Compared to the common cold is more serious often including fever and extreme fatigue
• Though some develop potentially lethal situations like
• Spreads very easily through respiratory droplets
• Prevented through annual vaccinations
• Vaccines can be useful up to six months
• Vaccine should also be updated every change of the flu/season to better match

Chicken Pox and Other Herpesvirus Infections

• Herpesviruses are a large group of viruses.
• Herpesviruses are characterised as once infected, the host never free of the virus
• The virus can become latent
• Symptoms:
  • sores around the mouth/genitals
  • blisters
  • rash
• Herpesvirues included include the following:
  • Varicella-zoster virus
  • Herpes simplex virus (HSV) types 1 and 2
  • Epstein-Barr virus (EBV)
• Epstein-Barr virus most often affects adolescents

Viral Encephalitis

• Possible cause, Inflammation of the brain tissue as a result of A viral infection.
• Can cause memory loss, delirium, nausea, fever, and headache.

Viral Hepatitis

• Is caused by three of the most common viruses and causes inflamed liver
• HAV
• HBV
• HCV
• There are effective vaccines for hepatitis A and B Symptoms: Include nausea, abdominal pain, diarrhea, fatigue, jaundice, and loss of appetite

Tattoos and Body Piercing

• Body piercing and tattoos have risks
• Needles could be used to transmit blood
• Tattoo ink should not get reused

Rabies

• Can Cause death potentially
• Found mostly among bats
• Most recent deaths can be traced back to bats, in relation to Canadian human rabies deaths

Poliomyelitis

• Leads to irreversible paralysis and affects that part of the nervous system
• Vaccine has shown to reduce dramatic decline
• Polio is a viral infection.

Human Papillomavirus (HPV)

• Causes non cancerous skin tumours, or warts
• Vaccine is an option

Fungi

• Is an organism that collects its food from organic matter
• Some cause fungal diseases, that are life threatening and severe
• C. albicans overgrowth can occur in other areas of the body, especially in the mouth in infants
• Prescription and OTC treatments are available.
• Includes yeast infections, skin fungi (Athlete's Foot), and systemic fungal infections.

Protozoa

• Millions of people in developing countries have protozoal infections.
• Giardiasis is rarely serious and can be treated with prescription medications.
• Infections caused by the protozoal include the following:
  • Trichomoniasis
  • Trypanosomiasis
  • Amoebic dysentery

Parasitic Worms

• Relatively cause infections that are mild
• Children have pinworm commonly
• Worm Infections often occur due to not paying close attention or being unhygienic
• Can develop from contaminated drink and food
• Parasitic worm infections include mild intestinal infections and pinworm (common in children).

Prions

Are proteinaceous infectious particles, their presence does not trigger the immune response. Known prion diseases include

• CJD a Creutzfeldt-Jakob disease
• BSE bovine spongiform encephalopathy disease in cattle
• scrapie disease that affects sheep
• CJD, BSE, and scrapie are prion diseases that do not trigger an immune response.

Emerging Infectious Diseases

• Represent a great challenge to nations in the future
• Exotic diseases have a low chance of the average Canadian contracting
• Zika, Ebola, and West Nile are emerging infectious diseases.
• COVID-19 is another emerging infectious disease (covered in depth elsewhere).