Both patients who were diagnosed with a CNS infection but had a CSF leukocyte below the threshold of 5/mm3 were HIV positive, and suffered from cerebral toxoplasmosis and progressive multifocal leukoencephalopathy (CD4 count respectively 120 and 34 × 10^6/l, viral load respectively 1984 and 17,600 copies/ml). The gene discussed is CD4; the disease is progressive multifocal leukoencephalopathy.