Fungal infection of brain tissue (Wikicommons, CDC). Some fungi persist in the human brain for years and begin to harm their host only in old age. What were they doing previously?
I've published a paper on manipulation of human behavior by fungal pathogens. Here's the abstract:
Many pathogens, especially fungi, have evolved the capacity to manipulate host behavior, usually to improve their chances of spreading to other hosts. Such manipulation is difficult to observe in long-lived hosts, like humans. First, much time may separate cause from effect in the case of an infection that develops over a human life span. Second, the host-pathogen relationship may initially be commensal: the host becomes a vector for infection of other humans, and in exchange the pathogen remains discreet and does as little harm as possible. Commensalism breaks down with increasing age because the host is no longer a useful vector, being less socially active and at higher risk of death. Certain neurodegenerative diseases may therefore be the terminal stage of a longer-lasting relationship in which the host helps the pathogen infect other hosts, largely via sexual relations. Strains from the Candida genus are particularly suspect. Such pathogens seem to have co-evolved not only with their host population but also with the local social environment. Different social environments may have thus favored different pathogenic strategies for manipulation of human behavior.
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Reference
Frost, P. (2020). Are Fungal Pathogens Manipulating Human Behavior? Perspectives in Biology and Medicine 63(4): 591-601. https://doi.org/10.1353/pbm.2020.0059