The first step in disease prevention is an understanding of disease etiology. In order for disease to occur, the triad of agent, host, and environment is necessary. For a disease agent to be transmitted from one animal to another it must first 'leave' the animal (route of transmission) and then it needs to 'enter' a susceptible animal (exposure route). For some diseases, the route of transmission and the exposure route are identical (shed or transmitted by aerosol and the susceptible animal is exposed via inhalation). For others, they are different (transmitted via feces and exposed via ingestion).

Since it is impossible to control all possible shedding (transmission) of disease agents, focusing efforts on limiting exposure wherever possible can help prevent disease.

The common disease exposure routes for animals and humans include aerosol/inhalation, direct contact, fomite, oral/ingestion, and vector-borne. Keep in mind that diseases may have multiple exposure routes and may be different for animals versus humans.

Designing prevention practices aimed at minimizing exposure through these five routes, rather than aimed specifically at hundreds of individual organisms, is the approach that will be reviewed in this module.

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