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Infectious Agents
Infectious agents (pathogens) include not only bacteria but also viruses, fungi, and parasites. The virulence of these pathogens depends on their number, their potency, their ability to enter and survive in the body, and the susceptibility of the host. For example, the smallpox virus is particularly virulent, infecting almost all people exposed. In contrast, the tuberculosis bacillus infects only a small number of people, usually people with weakened immune function, or those who are undernourished and living in crowded conditions.
Viruses are intracellular parasites; that is, they can only reproduce inside a living cell. Some viruses, such as HIV and hepatitis B and C, have the ability to enter and survive in the body for years before symptoms of disease occur. Other viruses, such as influenza and COVID-19, quickly announce their presence through characteristic symptoms.
Reservoir
A reservoir is any person, animal, arthropod, plant, soil or substance (or combination of these) in which an infectious agent normally lives and multiplies. The infectious agent depends on the reservoir for survival, where it can reproduce itself in such manner that it can be transmitted to a susceptible host.
Animate reservoirs include people, insects, birds, and other animals. Inanimate reservoirs include soil, water, food, feces, intravenous fluid, and equipment.
Portal of Exit
Portals of exit is the means by which a pathogen exits from a reservoir. For a human reservoir, the portal of exit can include blood, respiratory secretions, and anything exiting from the gastrointestinal or urinary tracts.
Once a pathogen has exited the reservoir, it needs a mode of transmission to transfer itself into a host. This is accomplished by entering the host through a receptive portal of entry. Transmission can be by direct contact, indirect contact, or through the air.
Transmission of respiratory infections such as COVID-19 is primarily via virus-laden fluid particles (i.e., droplets and aerosols) that are formed in the respiratory tract of an infected person and expelled from the mouth and nose during breathing, talking, singing, coughing, and sneezing. The competing effects of inertia, gravity, and evaporation determine the fate of these droplets. Large droplets settle faster than they evaporate and contaminate surrounding surfaces. Smaller droplets evaporate faster than they settle, forming droplet nuclei that can stay airborne for hours (becoming aerosolized) and may be transported over long distances (Mittal et al., 2020, July 10).
Human-to-human transmission of COVID-19 occurs primarily via three routes: (1) large particles that are expelled with sufficient momentum so as to directly impact the recipients’ mouth, nose, or conjunctiva; (2) physical contact with droplets deposited on a surface and subsequent transfer to the recipient’s respiratory mucosa; and (3) inhalation of aerosolized droplet nuclei delivered by ambient air currents. The first two routes associated with large droplets are referred to as the “droplet” and “contact” routes of transmission, whereas the third is referred to as “airborne” transmission (Mittal et al., 2020, July 10).