**1. Introduction**

Toxoplasmic encephalitis (TE), a life-threatening disease in HIV/AIDS infected individuals, is an inflammation of the brain caused by the reactivation of latent infection of the protozoa *Toxoplasma gondii*. Immunocompetent host when infected with *T. gondii* is almost always unnoticeable or develops mild and non-specific signs and symptoms, then tissue cysts are the consequence harbored in those infected persons life-long, quietly without any problem. However, when host immunity is suppressed by any cause but mostly by HIV/AIDS, the previously quiescent protozoa become active and the aggressive stage, tachyzoite, causes severe clinical manifestations in the Central Nervous System (CNS) such as encephalitis or abscess.

In the early 1980s, at the beginning of AIDS pandemic, there were many alarming casereports threatening the world medical community with increasing numbers of unknown causes and severe diseases presented in homosexual men, hemophiliacs and Haitian. TE was one of the most common opportunistic infections of this immunocompromised host. Huge efforts have been put on to combat with TE including budget, manpower and research on diagnostic methods, prophylaxis, treatment and prevention. The incidence of TE is now decreasing due to primary and secondary prophylaxis as well as immune restoration because of the HAART (Highly active antiretroviral therapy), but some old problems still exist and new ones have surfaced.

This chapter will focus on all aspects of TE including the etiologic organism, epidemiology, clinical manifestations, diagnostic methods, management and outcome as well as prophylaxis and prevention. Evidences from our research on *T. gondii* and literature review will be used as an input. With those frameworks, an extensive perspective on this fascinating disease will be forthcoming.
