**Abstract**

*Toxoplasma gondii* is a parasite that affects about 20–80% of the global population. Chronic infection with toxoplasma, also called latent infection, has largely been considered to be asymptomatic with minimal to no clinical effects or sequelae. Though there is now clear evidence in animal models and mounting evidence in humans that latent toxoplasmosis can have various effects on behavior, personality, cognition, and even psychiatric conditions. In this chapter, we will explore the role latent toxoplasmosis plays in the behavior of animals and humans, and discuss the possible mechanisms for the observed effects.

**Keywords:** toxoplasmosis, animal behavior, human behavior, personality, cognition, mechanism, neuropsychiatric conditions

## **1. Introduction**

Since its discovery in 1908, there has been curiosity about the interplay between toxoplasmosis, behavioral changes, personality changes, and cognitive changes in both animals and humans. Toxoplasmosis is one of the most common parasites globally. After primary infection with toxoplasmosis, it lives in a latent form usually in the nervous system and muscle tissues of the host. Animal studies demonstrate that latent infection with toxoplasmosis can have effects on the behavior and overall performance of animals. Human studies also suggest that latent toxoplasmosis infection affects personality, behavior, and cognition, and likely plays a role in the development of psychiatric conditions, particularly schizophrenia. It is hypothesized that modification of the host behavior helps to promote transmission of the parasite. Through multiple intricate experiments, the mechanisms of how latent infection affects behavior and psychomotor performance are felt to be largely driven by altered levels of dopamine. It has been also been postulated that humans with certain blood groups who are infected with *Toxoplasma* are three times more likely to be in a traffic accident while others are protected from the effects of toxoplasmosis infection. The ubiquitous nature of toxoplasma and easily accessible methods to determine infection, allows it to be an ideal parasite to study not only the manipulation theory in animal models but to better understand the subtle ways toxoplasmosis may also be affecting human behavior.
