Which of these infections is NOT commonly associated with AIDS?

Syphilis is not commonly associated with AIDS in the same way as the other infections listed. AIDS, caused by the HIV virus, significantly weakens the immune system, making individuals more susceptible to opportunistic infections and certain conditions.

Pneumocystic pneumonia, candidiasis, and toxoplasmosis are all hallmark infections or conditions that arise in individuals with advanced HIV disease or AIDS due to their compromised immunity. Pneumocystic pneumonia, for instance, is a major cause of pneumonia in AIDS patients, particularly when the CD4 cell count falls below a certain threshold. Candidiasis can present as oral thrush or esophageal candidiasis, signaling a weakened immune response. Toxoplasmosis, often resulting from reactivation of latent infection, poses serious risks as well when CD4 counts drop.

While syphilis can affect anyone regardless of HIV status, it does not occur as an opportunistic infection that typically signals advanced HIV or AIDS. Instead, syphilis is a sexually transmitted infection that can be treated effectively and does not directly indicate the immunocompromised state associated with AIDS. Thus, it is the infection that stands apart from the others in this context.