Apparently a common parasite that is present in about 40% of the world's population affects the behavior of people it has infected, and it does it in different ways depending on gender.
[url]http://www.smh.com.au/news/national/parasite-makes-men-dumb-women-sexy/2006/12/26/1166895290973.html[/url]
Edit: Wired news is now reporting on this too:
[url]http://blog.wired.com/tableofmalcontents/2006/12/the_rise_of_tox.html[/url]