As noted in Chapter 17, the HIV-1 (AIDS) virus may produce a mild aseptic meningitis lasting 2-3 weeks as the first clinical manifestation of infection. The exact incidence is unknown, but it is probably greater than the 2%-5% estimated in one report. The majority of patients do not manifest this stage but develop more advanced disease at some time in the future. Later in the disease, more than 30%-70% of patients develop symptoms of CNS infection. Some of these cases are due to superimposed infection by other organisms (Toxoplasma, Cryptococcus) rather than HIV alone. HIV infection of the brain is most often manifested by dementia, but more than 15% develop progressive focal leukoencephalopathy. There is relatively little information about CSF findings in this disorder. In one report, 27% had elevated protein levels and 14% had elevated WBC count, with all cell counts being less than 25/mm3 and with 80%-100% of the cells being mononuclear. The brain abnormalities are best shown by CT or MRI.