Upon entering the study, all participants take medication to treat toxoplasmic encephalitis. They undergo lumbar puncture (spinal tap) to obtain CSF for analysis, an FDG-PET scan, and a 201TI-SPECT scan. For the PET scan, a radioactive substance is injected into an arm, followed by scanning in a doughnut-shaped machine similar to a CT scanner. SPECT is similar to PET but uses a different radioactive tracer, and the patient lies on a table while the SPECT camera rotates around the patient's head. Patients whose test results indicate a low risk for lymphoma continue antibiotic therapy for toxoplasmosis. They have repeat MRI scans around 4, 7, and 14 days after starting the drug to monitor the response to therapy. Antiretroviral therapy is initiated in patients who are not already on such a regimen.