Infection in people who are immunocompromised often results from reactivation of latent disease, as seen in patients who receive hematopoietic stem cell transplants or in those with advanced HIV disease, and it commonly manifests as CNS disease with brain abscesses. Less common clinical presentations include pneumonitis, myocarditis, or chorioretinitis. Patients who receive solid organ transplants and who lack prior immunity to T gondii (IgG seronegative), however, are at particular risk of disseminated toxoplasmosis because the parasite can be transmitted in the transplanted organ. The highest risk occurs in those patients who receive transplants and who are Toxoplasma “mismatched” with the donor (the recipient is seronegative, while the donor is seropositive); untreated, 25% of such patients will develop clinical toxoplasmosis. In these patients, prophylaxis, most commonly with trimethoprim-sulfamethoxazole, will effectively prevent infection.