(NaturalNews) Writing in Scientific American, Stanford professor Robert Sapolsky has catalogued a number of well-established cases in which parasites and other infectious organisms are able to actually modify the behavior of their hosts.
Among the examples given in his article, "Bugs in the Brain," Sapolsky cites the rabies virus, which makes its hosts aggressive and more likely to bite. Because rabies is spread when the saliva of an infected organism enters the blood of another organism, this is how the virus ensures its reproduction. Rabies also increases the saliva production of its host's body, producing the characteristic foaming at the mouth.
The malaria parasite plasmodium exhibits a more complex control. When mosquitoes first contract malaria, the disease makes them more cautious and less likely to bite, giving itself time to reproduce in their guts. Then the plasmodium makes the mosquito bite longer and more frequently, often several times in a night, increasing the odds that the parasite will spread. Humans that have been infected with malaria give off signals making them more attractive to biting mosquitoes.
Many parasites make their hosts more reckless, or even suicidal. Certain trematodes that infect brackish water crustaceans cause their hosts to exhibit aberrant evasive behaviors that make it harder for them to escape from predators, and make them more likely to move toward the light where predators can find them. In this way, the hosts are eaten by birds, and the parasite can spread to another host through the bird's droppings.
Even more extreme, grasshoppers infected with the hairworm parasite become more likely to actually fling themselves into the water to their deaths, because the hairworm can only reproduce in water.
Among rodents, the organism toxiplasma gondii removes the inborn aversion to cat pheromones, making the rodents more likely to be eaten so that the parasite can spread via cat droppings. Humans infected with the parasite also exhibit behavioral changes, namely a decrease in "novelty seeking."
Researchers have noted that cultures in which the rate of toxiplasma infection is high tend to have higher rates of "masculine" sex roles, uncertainty, avoidance and neuroticism.