Toxoplasmosis

Main article: Toxoplasmosis
Behavioral modifications of the host

It has been found that the parasite has the ability to change the
behavior of rats and mice?making them less fearful of cats. This effect is
advantageous to the parasite, which will be able to sexually reproduce
if its host is eaten by a cat. [2] There has been speculation that human
behaviour may also be affected in some ways, and correlations have been
found between latent Toxoplasma infections and various characteristics
such as increased risk taking behavior, slower reactions, feelings of
insecurity, and neuroticism. [3]

Several independent pieces of evidence point towards a role of
Toxoplasma infection in cases of schizophrenia and paranoia. [4]

Alteration of human sex proportion A recent study [5] has indicated
Toxoplasmosis is also correlated strongly with an increase in boy births
in humans. According to the researchers, depending on the antibody
concentration, the probability of the birth of a boy can increase up to a
value of 0.72 ... which means that for every 260 boys born, 100 girls are
born. The study also notes a mean rate of 0.60 to 0.65 (as opposed to
the normal 0.51) for Toxoplasma positive mothers.