This photo shows HIV, the AIDS virus (yellow), infecting a human immune cell. The AIDS virus can genetically evolve and independently replicate in patients' brains early in the illness process, researchers funded by the National Institutes of Health have discovered. An analysis of cerebral spinal fluid (CSF), a window into brain chemical activity, revealed that for a subset of patients HIV had started replicating within the brain within the first four months of infection. CSF in 30 percent of HIV-infected patients tracked showed at least transient signs of inflammation - suggesting an active infectious process - or viral replication within the first two years of infection. There was also evidence that the mutating virus can evolve a genome in the central nervous system that is distinct from that in the periphery.