Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Pneumocystis Infections ?
    ### Response:
    Pneumocystis jirovec is a tiny fungus that lives in the lungs of many people. Most people's immune systems keep the fungus under control. But if your immune system is weak, the fungus can make you very sick.     The most common problem of infection is pneumocystis pneumonia (PCP).
    