Causes of infectious diseases

• Disease is any process or condition that impairs the normal functioning of a living organism or its parts.
• Infectious diseases are caused by pathogens.
• A pathogen is any organism capable of causing disease.
• An epidemic is a widespread outbreak of an infectious disease in a community at a particular time.
• Disease occurrence requires a pathogen, a susceptible host, a favorable environment for pathogen reproduction, and transmission between individuals.

Direct Transmission

• Direct transmission involves the transfer of a pathogen via exposure to infected skin or body secretions.
• Modes of direct transmission include touching, sexual contact, direct contact with blood or bodily fluids, and contact with wounds.
• Vertical transmission includes transplacental transmission, transmission during vaginal birth, and breastfeeding.
• HIV/AIDS is an example of a disease transmitted via sexual activity.

Indirect Transmission

• Indirect transmission involves the transfer of a pathogen to a new host via a non-living object.
• Infection occurs from a reservoir created by the host outside itself.
• A fomite is any object or substance that carries infection.
• Airborne transmission is difficult to control.
• Modes of indirect transmission include contaminated food or water, contact with infected surfaces (fomites), airborne transmission (coughing, sneezing), and improper sterilization of surgical equipment.
• Measles virus transmission occurs through infected droplets.
• Gastroenteritis, caused by E. coli, can be contracted via contaminated food/water.

Vector Transmission

• Vector transmission involves the transfer of a pathogen via another organism.
• The most common form is a bite from an arthropod that is bloodsucking and transmits the pathogen during feeding.
• Examples of vector-borne diseases include Hendra and Nipah virus
• Malaria is transmitted by mosquitoes (female Anopheles) that lay their eggs in water.

Koch and Pasteur

• The works of Koch and Pasteur disproved the theory of spontaneous generation.
• Spontaneous generation proposed that living organisms can arise from nonliving matter.
• Pasteur developed the 'germ theory of disease', stating that germs (microbes) cause disease and that all microorganisms come from pre-existing microorganisms.
• Koch demonstrated that Bacillus anthracis was the causative agent of anthrax.
• Koch used an agar plate to isolate Bacillus anthracis from the blood of livestock that died from anthrax, proving the disease was obtained from a pure culture of bacteria and revealed that a specific microbe caused the disease.
• Koch cultured the bacteria and inoculated it into mice, which then developed anthrax and died; blood samples of mice tissues identified anthrax bacteria.
• Mice that had not been inoculated didn’t develop anthrax.

Koch's Postulates

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

Pasteur's Contributions

• Pasteur is credited with creating the science of microbiology through experimentation.
• Pasteur identified microbes as responsible for spoilage during the production of wine, beer, and vinegar, leading to the development of pasteurization.
• Pasteur's 'swan-necked flask' experiment disproved spontaneous generation by showing that microorganisms from the air did not reach broth in swan-necked flasks.
• No bacterial/fungal growth was observed unless the flask was broken or tilted to allow the solution to contact the curve of the flask.
• Pasteur's work contributed to the development of vaccines for diseases like fowl cholera, based on the principle of immunity.

Pathogen Adaptations (Virulence Factors)

• Pathogens have adaptations, also know as virulence factors, that promote infection.

Prions

• Host B lymphocytes may secrete factors (e.g., TNF) that enable prions to invade follicular dendritic cells in lymphoid tissue.
• PrPSC can bind to the surface of neurons, leading to neuron synapse degradation.
• From lymphoid tissue, prions invade nervous tissue and travel to the brain.
• Prions may bind to other proteins such as ferritin to facilitate movement through the gut.

Viruses

• Viruses must enter the nucleus of the host cell to replicate the viral genome.
• Viral surface proteins adhere to host cell surface receptors and co-receptors, enabling attachment via complementary binding.
• Receptor-mediated endocytosis is a key cellular process for the movement of the virus into the cell.
• Enveloped viruses (e.g., influenza) are enclosed within an envelope (endosome) formed from the host cell membrane as they move into the cell.
• Non-enveloped viruses (e.g., polio virus) form a pore in the host cell membrane to deliver the viral genome.
• Some viruses use the cell’s normal membrane-forming processes, following a route through the endoplasmic reticulum and Golgi body before budding off from the surface.

Bacteria

• Pili or fimbriae enable binding with the host cells’ surface receptor proteins.
• Translocation of bacterial proteins causes the host cell membrane to engulf bacteria.
• Adhesins on the surface of the bacterial cell resist washing action of secretions like urine, mucus, and cilia.
• Bacteria form biofilms.
• Bacteria form endospores.
• Enzymes such as collagenase, hyaluronidase, and lecithinase break down cell contents.
• If phagocytosed, Listeria monocytogenes secretes haemolysin, which selectively destroys the phagosomal membrane but not the cell membrane
• Capsules resist phagocytosis by host cells.
• The host cell cytoskeleton is used for intracellular movement.
• Toxins are secreted to damage host cells (endotoxins and exotoxins).

Protozoa

• Surface proteins called antigens bind with sulfated glycosaminoglycan on the host cell’s surface.
• Micronemes secrete adhesive proteins to strengthen the attachment to the host cell for helical gliding.
• Helical gliding is performed by the protozoan to corkscrew itself through the cell membrane, facilitated by the actin–myosin motor complex.
• Granule proteins are released to form a vacuole that envelopes and protects the protozoan from lysosome digestion.

Fungi

• Cell wall or capsule molecules permit adhesion to host cells.
• Thermotolerance involves the synthesis of heat shock proteins to cope with body temperatures.
• Converts from saprophytic mycelium to parasitic yeast when exposed to heat (dimorphism).
• Cell walls and capsules offer fungi protection from host attack.
• Hormone receptors for 17B-oestradiol on fungal cells may change the incidence of certain fungal diseases between men and women.
• Secretion of hydrolytic enzymes causes damage to host cells and provides nutrients for fungus.
• Evasion mechanisms include capsule production, suppression of cytokine production by host cells, and reduced fungicidal power of macrophages.
• Opportunistic fungal infections (e.g., Cryptococcus neoformans) are common in immunosuppressed patients (e.g., HIV/AIDS, chemotherapy, lymphoma).

Macroparasites

• Hookworms secrete immunomodulatory proteins that reduce host cell immune responses.
• The third larval stage (L3) in soil invades hosts via hair follicles and migrates through circulation to lungs, trachea, and intestines.
• Teeth in the buccal capsule anchor the worm to the gut.
• Ticks' highly specialized mouthparts are inserted into host skin to attach and are anchored by 'attachment cement'.
• Biologically active molecules are secreted in saliva to prevent vasoconstriction, clot formation, and inflammatory responses.

Adaptations to Facilitate Transmission

• Pathogens develop adaptations depending on their method of transmission.

Airborne

• Adaptations include the ability to remain suspended in air for long periods, resistance to drying out, and causing sneezing and coughing to facilitate transmission.
• Adaptation to tolerate a wide range of oxygen concentrations.
• Influenza viruses use this method.

Water-borne

• Adaptations include the ability to colonize and proliferate in water, modified outer surface structures (e.g., fimbria, flagella) for motility, and halotolerance for marine organisms.
• Many are not destroyed by simple boiling of water or other water-treatment processes.
• Examples include Legionella, Vibrio, Giardia, and Campylobacter spp.

Faeco-oral

• Adaptations include stability in varied environments such as the stomach, low oxygen of large intestine and antimicrobial resistance genes.
• Induction of vomiting and diarrhea increases likelihood of transmission.
• Examples include E. coli and Salmonella spp.

Vector-borne

• Pathogens have a structural integrity and capacity to act as an infectious organism or agent when absorbed by or bound to a vector.
• Surface receptor proteins can attach to the vector (pili and adhesion proteins)
• Examples include Zika virus, Malaria and Dengue fever.

Sexual Transmission

• Adaptations include the ability to enter the uterus and survive in the placenta.
• Examples include chlamydia, HIV/AIDS virus, gonorrhea, and herpes simplex virus.