Over the last few decades, a new pathology has appeared, directly related to the modified immune status of the hosts. It presents several distinctive points. The central nervous system is particularly affected. The opportunistic pathogenic agents do not usually injure the brain parenchyma and are not known for their aggressiveness in normal adults (papovavirus). Diagnosis of these different diseases is often difficult, some biological tests being irrelevant because of alterations of the immune system (toxoplasmosis). The lesions may be exclusively located in the brain (tuberculosis, lymphomas) which is not usually affected by these agents. Response to therapy is frequently poor, the clinical course being rapid and fatal. However, therapy may be successful in some cases, justifying the use of somewhat aggressive procedures (biopsy) in order to obtain an accurate diagnosis. It is important to be aware of these data, and an understanding of them may help in managing these already difficult patients. They also make possible some interesting pathogenic hypotheses.