[Pneumocystis carinii pneumonia in AIDS. New concepts for an old problem].

Pneumocystis carinii (PC) is an ubiquitous pathogen phylogenetically considered a fungus. In individuals with T-cell deficiency, PC produces typically an interstitial pneumonia. The primary infection is, perhaps, transmitted airborne, and is acquired during early infancy. PC was a rare cause of disease until the advent of AIDS. In susceptible patients infected with HIV it remains a major cause of morbidity and mortality despite appropriate prophylaxis and treatment. Mutations in the gene that encodes the enzyme dihydropteroate synthase are seemingly accountable for the failure of prophylaxis in some individuals. The incidence of new cases of PC pneumonia (PCP) in patients with infection with HIV has dropped substantially since the advent of highly active antiretroviral therapy (HAART). Ongoing trials are designed to study the effect of withdrawing prophylaxis for PCP in patients whose T-cell count has risen over 200/mm3 in response to HAART.