Infectious disease transmission cycles

A further point should be made at this stage of the chapter. There is a great diversity of types and biological modes of infectious diseases. The simplest transmission cycle is one where a pathogen is transmitted (by "contagion") from an infected person to another susceptible person directly (e.g. via droplet secretion or sexual contact). Cycles of medium complexity are those where the pathogen is transmitted indirectly (through an intermediate plant, animal, or environmental factor such as water). Vector-borne pathogens - those that rely on a vector (such as a mosquito, fly, cockroach, tick or rodent) to infect humans - are a major subset of this second group.

Pathogens that are human-adapted are the anthroponoses. They circulate from human to human, either with (e.g. malaria) or without (e.g. cholera) the intervention of vector species. The zoonoses are pathogens that naturally infect (but do not necessarily affect) non-human animal species (the reservoir host), and which occasionally infect "bystander" humans. These too may have an intervening vector (a mosquito, as in West Nile Virus) or not (such as direct transmission, as in rabies from the bite of a dog). Some of these cycles are highly complex, with more than one animal reservoir needed in the transmission cycle (e.g. tick and deer, Lyme spirochete) or with numerous different species capable of acting as reservoirs (in the case of Ross River virus). Figure 14.1 summarizes the main types of transmission cycles for infectious agents.

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