Parasitic organisms can influence animal behavior
It is well known that parasitic organisms can influence animal behavior, typically in a way which helps the parasite to reproduce. The rabies virus, for example, induces aggressive biting behavior in its victims, which allows the virus to pass from the rabies-victim's saliva into the bite-victim's bloodstream. Within the animal kingdom, there are other examples of less deadly diseases which cause clear changes in behavior pattern.
There is no compelling reason to think that behavior patterns in humans might not be similarly affected by non-deadly microorganisms, causing particular individuals or groups to act in seemingly irrational ways without any obvious health effects. These changes in behavior might, over time (decades or centuries), have become an accepted norm for certain infected groups and explained away as being part of the shared culture unique to those groups. This is, however, purely a conjecture without supporting evidence as of yet.
- 2007-04-03 How a parasite can help a cat catch its mouse by Jonathan M. Gitlin