Toxoplasmosis is a parasitic disease caused by infections of the protozoan parasite Toxoplasma gondii.

It is extremely widespread in the human population. Infection rates vary in various regions of the world - but it's currently estimated that about 40 - 60% of the global human population permanently hosts the parasite.

The parasite itself was first identified in 1908, and the first case of Toxoplasmosis in 1940. The organism and the infection routes are now very well understood. Evidence of infection has now been found in almost all warm-blooded animals. Cats are the known to be one of the main host species.

It usually causes no physical symptoms in those infected - other than a short flu-like episode in some individuals. However, in the early 2000s it was suggested that the parasite may also have a psychological effect on those infected.

It has now been linked to changes in brain dopamine levels (up to 15%) and can lead to so-called 'aberrant' behaviour - which to date has been verified in rats, otters and cats.

The mechanism of action that the parasite appears to have in relation to altering brain chemistry is unknown.

Some research groups are now suggesting that it may have an influence on the prevalence of Schizophreniaplugin-autotooltip__plain plugin-autotooltip_bigSchizophrenia

"Schizophrenia is a mental disorder characterized by abnormal social behavior and failure to understand what is real. Common symptoms include false beliefs, unclear or confused thinking, auditory and visual hallucinations, reduced social engagement and emotional expression, and a lack of motivation." and Obsessive-Compulsive disorderplugin-autotooltip__plain plugin-autotooltip_bigObsessive-Compulsive disorder

Obsessive–compulsive disorder (OCD) is a mental condition in which a person feels the need to perform certain routines repeatedly (called "compulsions"), or has certain thoughts repeatedly (called "obsessions"). The sufferer is unable to control either the thoughts or activities for more than a short period of time.^{[ ref needed ]}

Bearing in mind that around half the human population may be infected, it's possible that it may even be having an effect at the 'societal' level. See :

Can the common brain parasite, Toxoplasma gondii, influence human culture? Proc Biol Sci 2006 Nov 7;273(1602):2749-55

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