Infection occurs when a biologic agent invades the host and multiplies, producing a change within that host. This change may be structural, functional, or biochemical, is not necessarily harmful, and could be limited to something as benign as a serologic response. The consequences to exposure to an infectious agent would be:
1. No infection or disease
2. Infection (unapparent, latent, subclinical)
3. Disease (mild to severe to death)
A carrier is an individual shedding the infectious agent without showing clinical illness. There are many different types of carriers including:
* healthy incubationary convalescent
* intermittent chronic
Healthy carriers are probably the most important from the standpoint of disease transmission. These animals shed the agent but are never recognized as being clinically ill. In some instances, the agent may persist within the body for a long period of time with intermittent shedding at periods of stress.
As an example, in Salmonella typhimurium infections in swine, the agent can be isolated from the mesenteric lymph nodes for as long as 6 months with intermittent shedding following stress situations over this period of time.