What is the typical pneumonia associated with AIDS caused by?

The typical pneumonia associated with AIDS is primarily caused by Pneumocystis carinii, which is now more accurately referred to as Pneumocystis jirovecii. This organism is a opportunistic pathogen that often affects individuals with compromised immune systems, specifically those with a CD4 T lymphocyte count below 200 cells/mm³, which is characteristic of AIDS.

Individuals with AIDS are particularly vulnerable to P. jirovecii pneumonia (PCP) because their immune response is weakened, and this organism can produce severe pneumonia that can be life-threatening if not treated properly. PCP presents with symptoms such as progressive dyspnea, nonproductive cough, and can lead to significant respiratory distress.

Other pathogens listed can also cause pneumonia in the general population or in different contexts but are not the primary concern in patients with AIDS. For example, Streptococcus pneumoniae is a common cause of community-acquired pneumonia, Mycobacterium tuberculosis causes tuberculosis, and Histoplasma capsulatum is associated with fungal infections, typically in immunocompromised patients. However, none of these are as specifically associated with AIDS-related pneumonia as Pneumocystis jirovecii.