Toxoplasma infection is so pervasive and influential that it has been fingered as a possible contributer to differences between cultures: <a href="http://scienceblogs.com/notrocketscience/2008/10/toxoplasma_the_brain_parasite_that_influences_human_culture.php" rel="nofollow">http://scienceblogs.com/notrocketscience/2008/10/toxoplasma_...</a>