FIGURE 1: T. gondii stage conversion and its regulation by the host (cell) microenvironment. Within one to two weeks of infection, highly active tachyzoites (two parasitophorous vacuoles within human foreskin fibroblasts are depicted; left micrograph) convert to relatively dormant bradyzoites (a tissue cyst from mouse brain containing hundreds of individual bradyzoites is shown; right micrograph). Formation of long-lived bradyzoite-containing tissue cysts is crucial for parasite transmission to new hosts. Reconversion of bradyzoites to tachyzoites can occur in immunocompromised individuals (e.g. those with AIDS or transplant recipients (TPX)) leading to life-threatening disease. Distinct characteristics of the infected host cell or of bystander cells can trigger or inhibit differentiation towards the bradyzoite stage as indicated in the upper part of the figure. Noteworthy, parasite-intrinsic triggers may also govern stage differentiation (not depicted). See main text for further details.