A pair of authors suggest that the rise in mental illness since the 18th century is due to biological factors, possibly a virus: “Growing population and urbanization allowed the infectious agent — a virus or parasite — to spread more rapidly in the densely populated cities, they say.” Or it could just be increased diagnosis, rather than increased occurrence.
Seattle PI: Virus behind insanity, authors suggest
(via Post Atomic)
See also: Is Schizophrenia Caused by Retroviruses?