**Core Concept**
Opportunistic infections are caused by microorganisms that typically do not cause disease in a healthy individual but can take advantage of a compromised immune system to cause infection. These microorganisms are often ubiquitous and can be found in the environment, but they are not pathogenic in the absence of immunosuppression.

**Why the Correct Answer is Right**
The correct answer is an example of an opportunistic pathogen that can cause disease in individuals with weakened immune systems, such as those with HIV/AIDS, cancer, or taking immunosuppressive therapy. This microorganism can infect various parts of the body, including the lungs, brain, and gastrointestinal tract, and can cause a range of symptoms from mild to severe.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not an opportunistic pathogen. Instead, it is a common cause of food poisoning and is typically associated with a strong immune response.
**Option B:** This option is incorrect because it is not an opportunistic pathogen. Instead, it is a common cause of respiratory infections and is typically associated with a strong immune response.
**Option C:** This option is incorrect because it is not an opportunistic pathogen. Instead, it is a common cause of skin infections and is typically associated with a strong immune response.

**Clinical Pearl / High-Yield Fact**
It's essential to recognize that opportunistic infections are a significant concern in patients with compromised immune systems, and prompt diagnosis and treatment are crucial to prevent morbidity and mortality.

**Correct Answer:** Pneumocystis jirovecii. Pneumocystis jirovecii is a fungus that causes opportunistic pneumonia, particularly in individuals with HIV/AIDS.