TASK 1

Definition of Disease

• Disease: any process or condition that adversely affects the normal functioning of a living thing or parts of it.
• Infectious disease: caused by another organism or an infective agent known as a pathogen.
• Pathogen: organism capable of causing disease.
• Epidemic: a widespread occurrence of an infectious disease in a community at a particular time.

Conditions for Disease Occurrence

• Pathogen must be present.
• A susceptible host must be available.
• An environment favorable for pathogen reproduction and replication is necessary.
• Transmission between affected and unaffected individuals.

Modes of Transmission: Direct

• Transfer of pathogen via exposure to infected skin or body secretions.
• Occurs through touching, sexual contact, direct contact with blood or other bodily fluids, and direct contact with wounds.
• Vertical transmission: transplacental, during vaginal birth, or breastfeeding.
• Example: HIV/AIDS transmitted via sexual activity.

Modes of Transmission: Indirect

• Transfer of the pathogen to a new host via a non-living object.
• Infection occurs from a reservoir created by the host outside itself.
• Fomite: any object or substance that carries infection.
• Airborne: difficult to control.
• Occurs through contaminated food or water.
• Contact with infected surface (fomite).
• Airborne transmission via coughing or sneezing.
• Improper sterilization of surgical equipment.
• Examples: measles virus from infected droplets and gastroenteritis caused by E. coli from contaminated food/water.

Modes of Transmission: Vector

• Transfer of the pathogen via another organism, such as an arthropod.
• Most common: bite from a bloodsucking arthropod that transmits the pathogen during meals.
• Examples: hendra and Nipah virus.
• Malaria is transmitted by mosquitoes (female Anopheles) that lay their eggs in water.

Disproving Spontaneous Generation

• Works of Koch and Pasteur disproved the theory of spontaneous generation, which stated that living organisms can arise from nonliving matter.
• Pasteur: 'germ theory of disease' states that germs (microbes) cause disease and all microorganisms come from pre-existing microorganisms.
• Koch's investigations proved that Bacillus anthracis was the causative agent of anthrax.
• Agar plate used to isolate Bacillus anthracis from the blood of livestock that died from anthrax to show the disease was obtained from a pure culture of bacteria.
• Revealed that a specific microbe caused the disease.
• Koch cultured the bacteria and then inoculated it into mice, which developed anthrax and died.
• Anthrax bacteria was identified in blood samples of mice tissues.
• Mice that had not been inoculated did not develop anthrax.

Koch's Postulates include:

• The same microorganism must be present in every diseased host.
• The microorganism must be isolated and cultured in the laboratory, accurately described, and recorded.
• When a sample of the pure culture is inoculated into a healthy host, that host must develop the same symptoms as the original host.
• The microorganism must be able to be isolated from the second host, cultured, and identified as the same as the original species.

Pasteur's Contributions

• Pasteur created the science of microbiology through experimentation.
• Microbes are agents responsible for spoilage during the production of wine, beer, and vinegar, leading to the development of pasteurization.
• 'Swan-necked flask' experiment disproved spontaneous generation.
• Meat broth boiled in flasks with long, drawn-out necks that were not sealed.
• Microorganisms from the air did not reach the broth as they were trapped in the narrow neck and curve of the glass.
• No bacterial/fungal growth observed.
• Bacterial growth occurred in broken flasks exposed to the air.
• Tipping flask to allow solution to curve resulted in bacterial growth.
• Theory of spontaneous generation discredited.
• Pasteur's work contributed to the development of vaccines for diseases such as fowl cholera, based on the principle of immunity.

Adaptations of Pathogens: Prions

• Host B lymphocytes are thought to play a role by secreting factors that enable prions to invade follicular dendritic cells in lymphoid tissue.
• PrPSC binds to the surface of neurons, causing neuron synapse degradation.
• From lymphoid tissue, prions invade nervous tissue through autonomic nerves and travel to the brain.
• Prions may bind to other proteins such as ferritin to facilitate movement through the gut.

Adaptations of Pathogens: Viruses

• Must enter the nucleus of the host cell to facilitate replication of the viral genome.
• Viral surface proteins adhere to host cell surface receptors and co-receptors.
• Protein coats possess surface proteins that enable it to attach to host cells via complementary binding.
• Some may possess a lipid membrane that surrounds the capsid.

Adaptations of Pathogens: Bacteria

• Pili or Fimbriae binding with the host cells’ surface receptor proteins that are matching.
• Translocation of bacterial proteins causes host cell membrane engulfment of bacteria.
• Adhesins on the surface of the bacterial cell resist washing action of secretions such as urine, mucus, and cilia.

Adaptations of Pathogens: Protozoa

• Surface proteins called antigens are able to bind with sulfated glycosaminoglycan on the host cell’s surface.
• Micronemes (organelle) secrete adhesive proteins to strengthen attachment to the host cell.

Adaptations of Pathogens: Fungi

• Assisted by cell wall or capsule molecules that permit adhesion to host cells.
• Thermotolerance – heat shock proteins are synthesized to cope with body temperatures (higher than air temperature).

Adaptations of Pathogens: Macroparasites

• Hookworms: They can secrete immunomodulatory proteins that reduce host cell immune responses.
• Ticks: Have highly specialized mouthparts that are inserted into the host skin to attach.

Adaptations for Transmission: Airborne

• Able to remain suspended in air for long periods and resist drying out.
• Pathogen causes sneezing and coughing, which causes ejection and transmission to the new host.

Adaptations for Transmission: Water-borne

• Able to colonize and proliferate in water, so environmental reservoirs are present (e.g., from fecal material).
• Modified outer surface structures (e.g., fimbria, flagella) allow motility.

Adaptations for Transmission: Faeco-oral

• Pathogens are generally very stable in varied environments, e.g., acid in the stomach, low oxygen of the large intestine.

Adaptations for Transmission: Vector-borne

• The pathogen's structural integrity and capacity to act as an infectious organism or agent are not affected when absorbed by or bound to a vector and survive inside their bodies.

Adaptations for Transmission: Sexual

• Ability to enter the uterus.
• Able to survive in the placenta and transmit disease when organisms consume the placenta.

Lines of Defence: Innate Immunity System

• Consists of physical and chemical barriers to pathogens.
• Initiated by chemical signaling from damaged tissue and consists of the inflammatory response and phagocytosis by WBCs.

Lines of Defence: Adaptive Immunity System

• Specific defense by lymphocytes in response to chemical signals.

Antigens

• Molecules recognized by the host as foreign that initiate the immune response.
• Exogenous antigens: Found on the invading pathogen.
• Endogenous antigens: Found within the toxic chemicals produced by pathogens.

Lymphatic System

• Filters and returns intercellular fluid to the blood using lymph nodes connected by lymph vessels.
• Lymph nodes filter and trap antigens, storing B and T lymphocytes.

Complement System

• A group of around twenty soluble proteins that assist other defense mechanisms in destroying extracellular pathogens.
• Proteins stimulate phagocytes to become more active, attract phagocytes to the site of infection, or destroy the membranes of the invading pathogen.

Innate Immunity: Physical Barriers

• The skin is a large physical barrier that protects cells from pathogens in the surrounding environment.
• Mucous membranes have cell junctions between epithelial cells that anchor them and restrict access to pathogens.

Innate Immunity: Chemical Barriers

• Urine has antimicrobial peptides (AMPs) that prevent bacteria from binding to epithelial cells and lyse bacterial cells.
• Sebum and sweat waterproof and lubricate the skin, hindering bacterial growth.

Innate Immunity: Second Line of Defense

• Inflammation: When a cell is invaded by a pathogen → chemicals are release which trigger vasodilation and increase vascular permeability
• Phagocytosis: Where phagocytes (specialized WBCs) envelope or enclose a non-specific antigen
• Apoptosis: Neighbouring cells die to form a wall of dead cells surrounding infected cells, forming a capsule structure known as granuloma
• Pyrexia: Elevates body temperature to limit pathogen growth and enhance WBC activity.
• Cytokines: Are chemical messengers released by cells in response to pathogens (interleukin, interferon)

Adaptive Immunity System

• It does not respond to the pathogens directly. It responds to the antigens produced by the pathogens which evaded the innate immune system.
• Lag phase for first exposure to pathogens.
• Immunological memory creates primary response for future threats by the same pathogen.

Cell Mediated: T-lymphocytes

• Specialised WBC, manufactured in bone marrow & matured in thymus gland Types:
• Helper T cells: release chemicals that activate the cloning of cytotoxic T cells and B cells Bind to infected cells and destroy them
• Cytotoxic T cells: remain in the body to respond to secondary immune response
• Memory T Cells: regulate immune response when the infection has been defeated by signalling all cytotoxic T cells to be destroyed

Humoral Repsonse: B-lymphocytes

• Manufactured and matures in the bone marrow & possess specific surface body protein that can recognize and is specific to a particular antigen
• Antibodies: Proteins that are produced by the plasma cells and attach to antigens to destroy Strategies used:
• – antibodies bind to and coat pathogens, blocking their activity
• – neutralised pathogens clump together and are surround by thousands of antibodies

Cell-mediated immunity

• Pathogens engulfed by macrophages, which then display the antigen attached to their MHC II molecules
• Activated T cells clone and differentiate that migrate to site of infection and bind to antigens on infected cells, releasing cytokines that destroy infected cell and pathogens within +Memory T cells: remain in the body, and when exposed, they rapidly clone themselves to prevent the body from suffering the symptoms

Adaptive Immunity Response

• Adaptive response: Is when the body's immune response encounters an antigen for the first time (the body learns to recognize the antigen)
• Adaptive response: Is the reaction of the immune system when it contacts an antigen for the second and subsequent times.

Adaptive immune System Response

• Adaptive immune System Response: Is the presence of the complement system within the innate immune system. Example:
• Cholera: Production of cytokines, neutrophils, and AMPs kill bacteria or induce inflammation (innate response)
• Fungi: Most infections are prevented by thick keratin layer of the skin (Fungal infections are mostly prevented by barriers such as the thick keratin layer of the skin.)

Virus Response

• The ability for some viruses to block MHC I molecules from migrating to cell surface

Protozoa Response

Local/Regional COVID-19 Response

• Local: Immunisation programs implement by local government result in differences in vaccination.
• Regional: Strictness of the COVID quarantine guidelines vary across different regions.

Global COVID-19 Response

• Strictness of the COVID quarantine guidelines vary across different regions.
• Dengue Fever: Tropical disease mosquito-borne virus that causes fever, headache, joint pain, and rashes.
• May lead to Dengue Hemorrhagic Fever or Dengue Shock Syndrome.

Dengue Fever Incidence

• Current global estimates are that 3.9 billion in 128 countries are at risk of infection

Dengue Fever Prevalence

• Found in tropical and sub-tropical climates where the number of cyclical epidemics increase the expected rate of infection