**Core Concept**
Pneumocystis carnii (now known as Pneumocystis jirovecii) is a fungus that primarily affects individuals with weakened immune systems, particularly those with HIV/AIDS. The underlying principle being tested is the knowledge of opportunistic pathogens and their preferred hosts.

**Why the Correct Answer is Right**
P. jirovecii is a well-known opportunistic pathogen that primarily causes Pneumocystis pneumonia (PCP) in immunocompromised individuals, especially those with HIV/AIDS. The fungus takes advantage of the host's weakened immune system to cause infection.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while P. jirovecii can infect other immunocompromised individuals, it is not the primary population.
**Option B:** Incorrect as it is not the correct answer.
**Option C:** Incorrect as P. jirovecii primarily affects humans, not animals.

**Clinical Pearl / High-Yield Fact**
P. jirovecii pneumonia is a classic opportunistic infection in HIV/AIDS patients, and its presence is often used as an indicator of AIDS.

**Correct Answer:** D. Humans, particularly those with HIV/AIDS.