A recent study conducted by the U.S., Britain, and Czech Republic has concluded that toxoplasma gondii - carried in felines - may be responsible for shifts in personality and behaviour in humans.
Czech scientist Jaroslav Flegr of Charles University in Prague has stated that the virus makes women more reckless and promiscuous, while increasing reclusiveness, jealously, and sloppiness in men. It apparantly also increases your risk of being in an auto accident (either while driving a car or being struck by one) by 2.6 times.
Such sweeping statements without much direct connection to the virus itself have left most scorning the study, and an alternate view can be read here.