Carl Zimmer posts about how a parasite present in half the human population may be altering our psychology.
The parasite's ultimate hosts are cats, and it uses rats as an intermediate host. While it doesn't harm rats directly, the parasite does have the effect of making rats bolder and less afraid of cats, and therefore more likely to be eaten by cats.
Human contact with cats have made us an unintended intermediate host. Some preliminary research suggests that our psychology may be affected as well. Infected women tend to be more "outgoing and warmhearted", while men were more jealous and insecure (I grew up with cats).
The existential part of this comes from my experience many years ago in Southeast Asia when I contracted typhoid, a very unpleasant disease. (No, I'm not a carrier.) The drug used for treating me was chloramphenicol, a drug that is rarely used today, and it appeared to have some unpleasant psychological side effects. Besides making me depressed, I was extremely nervous, jumping at the slightest noise.
So a thought experiment - although it is very unlikely, let's suppose that the drug treatment temporarily eliminated a parasite that had always been present in my brain, and that short period of depression and nervous anxiety constitutes my "uninfected" personality. If that's the case, I think my sense of self is one that identifies with the infected, combined organism, rather than the single uninfected human.
Maybe some of the human/parasite melanges, especially the female ones (given the apparent positive effects), would agree with me.