FIG 1.

Microbial infracommunity relationships in space and time and their impact on host immunological variation. Hosts can be exposed to multiple parasites that are nearly identical or very diverse (A), and they can superinfect (similar strains) or coinfect (different parasite species) the host at the same time or occupy the host at different times. Even if they do occupy the host at different times, the primary infection can impose an historical contingency effect through its influence on immune system maturation and investment. For example, primary exposure can prime the host immune response to respond more quickly and with greater magnitude upon secondary infection (B) (black solid line). On the other hand, polarization of the immune response during exposure to the first parasite can negatively feed back on an arm of the immune system necessary for defense against the second parasite (green dashed line), resulting in dampened induction of an immune response relative to a host that was not previously exposed. Gray bars highlight the importance of infection history for interpreting immunological variation even in hosts that are not infected at the time of sampling.