Pneumocystis jiroveci pneumonia (formerly called Pneumocystis
carinii or PCP) is the most common opportunistic infection that
occurs in AIDS patients. Originally, researchers thought a one-
cell organism (protozoan) called Pneumocystis carinii caused the
disease, but recent research suggests that a fungus called
Pneumocystis jiroveci causes the pneumonia. However, the
condition is still commonly referred to as PCP. According to the
U.S. Centers for Disease Control and Prevention (CDC), PCP is
considered an AIDS-defining illness. This means that when HIV-
infected patients develop PCP, their condition has progressed to
AIDS. Even if healthy individuals are exposed to Pneumocystis
jiroveci, they do not typically develop PCP because they are
able to inhibit the growth of the fungus. However, children and
people with weakened immune systems, such as HIV/AIDS patients,
are unable to adequately fight against the fungus. Individuals
with a CD4 cell (helper T-cells that help fight against disease
and infection) count l