For the establishment of an infection and its eventual spread in a population certain conditions should be present. 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)
4. Immunity -
For any of these outcomes, the contributions by each of the Host, Agent and Environment must be considered. Let us briefly review these.First though, let us look at some definitions.
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.
Stages of infection
The period of infection can be described as a succession of different stages of host parasite interaction. The exact timing of each stage varies with the different infectious agents. Infection begins with exposure. At this point, the given agent invades the host in sufficient numbers to constitute an infective dose. In other words, the number of agents exceeds the infection threshold of the host. The agent then multiplies within the host until its numbers reach an escape threshold at which time the agent may overflow from the host. This stage is followed quickly by the period of communicability. This begins when the agent overflows in sufficient numbers to be infective for a new host. The number has reached the communicability threshold. In some instances, the escape threshold and communicability threshold coincide.
The number of infective units continues to rise within the host until the host defenses have been mobilized to the point where they reverse the curve. The number of infective units then drops below the threshold of communicability and the threshold of escape. Generally, this downward slope is greater for viral than for bacterial infections.