Sometimes real science is stranger than fiction. Stanford Professor Robert Sapolsky describes a number of examples of parasites or infectious microbes that modify behavior in a Scientific American review article titled "Bugs in the Brain" (pdf)*. Some examples from the review and recent scientific literature:
• The rabies virus increases saliva production and makes the infected host aggressive. When a rabid animal bites a host the virus is spread via saliva in the wound.
• Toxiplasma gondii causes infected rodents to specifically lose their inborn aversion to cat pheromones. This behavior is beneficial to toxiplasma, because it sexually reproduces in cats that have eaten infected mice and rats (original article). Infected cats in turn spread toxiplasma through their droppings. People infected with toxiplasma also exhibit behavioral changes, particularly a decrease in "novelty seeking". It's been proposed that toxiplasma infection has actually changed human culture, since there is a correlation between countries with a high rate of toxiplasma infection and increased neuroticism, uncertainty avoidance, and "masculine" sex roles.
• Grasshoppers infected with the hairworm (Spinochordodes tellinii become more likely to jump into water where the hair worm reproduces. The parasite essentially makes its host suicidal to further its own reproduction.
• Some trematodes that infect the brackish water crustacean, gammaridean anthropod cause changes in behavior that make the hosts more likely to move towards light and exhibit aberrant "suicidal" evasive behaviors. These behavioral changes make the infected crustacean more likely to be eaten by birds, which the trematode uses as a host for the next stage in its life cycle (pdf).
• Plasmodium, the cause of malaria, affects both its mosquito and animal hosts. Mosquitoes that drink plasmodium-infected blood initially become more cautious about finding another victim, giving plasmodium time to replicate. Once the plasmodium is infective, mosquitoes become more likely to bite more than one person in a night, and spend more time drinking blood. In turn, once a person is infected with plasmodium, he become more attractive to mosquitoes, continuing the life cycle of the parasite. (See "Malaria Parasite Makes You More Attractive (To Parasites)" New York Times, August 9, 2005). Plasmodium can also affect the nervous system. Infection of juvenile canaries with plasmodium affects the song control pathway in the brain, resulting in simpler songs as adults. (Pubmed).
Of course this research is fertile ground for science fiction. Parasites are often used as a crude form of brain control; the brain-controlling parasites in The Wrath of Khan or the brain slugs in Futurama, are examples of this.
Personally, I prefer microbes with more subtle and interesting behavioral effects. An example of such a story is David Brin's, "The Giving Plague", in which a virus that causes altruism infects the human population. You can read "The Giving Plague" on David Brin's web site.
What other influences might parasites and microbes have had on the human species? For speculation on the possible effects of retrovirus infection on human evolution, see Couturnix's musings on Greg Bear's Darwin's Radio and Darwin's Children, asking "Did a virus make you smart?".
The possibilities are really endless.
* For a more technical review, see Thomas et al. "Parasitic manipulation: where are we and where should we go?" Beav. Proc. 68: 185-199 (2005) (pdf)
(For cool photos, check out the CDC's parasite image library.)
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