The microbiological milieu that surrounds us assures that we are in contact with microorganisms continuously. These organisms, following their basic imperatives to grow and reproduce, achieve these goals on occasion via contact, invasion and infection of humans. However, the fact that humans exist largely without continuous microbial invasion and infection, despite the huge numbers of microorganisms surrounding them, suggests that there is more to the pathogenesis of bacterial invasion and infection than simple contact. Contact between microorganisms and the host can result from contact with other humans or animals already infected or colonized.
Infection can spread by either horizontal or vertical mechanisms.
Characteristic routes of exposure are repeatedly utilized by specific microorganisms. For example, illness occurring in many people following a large social gathering could potentially be the result of either respiratory or food-borne spread. Spread of enteric illnesses in child care centers is typically due to fecal-oral contamination. Winter months are typically the times of highest rates and widest spread of respiratory illnesses because of crowding indoors. The inoculum size of the organism making contact with the host may greatly alter the likelihood of developing infection successfully.