Here’s an excellent question that I somehow ignored. I’ll answer it with an excerpt from my book:
An “opportunist” is a person who takes advantage of opportunities, usually at the expense of others, for his own benefit. Similarly, an opportunistic infection (OI) is one in which a pathogen (a bacterium, virus, fungus, or parasite) takes advantage of a weakness in the body’s defense mechanisms to cause disease. In the case of HIV infection, an OI is usually an infection caused by an organism that is normally kept in check by the cellular immune system, the part of the immune system that is most damaged by the HIV virus.
Some pathogens are exclusively opportunistic, meaning that they almost never cause problems in people with normal immune systems. Examples include many of the common HIV-related OIs, including Pneumocystis, Cryptococcus, Mycobacterium avium complex (MAC), and Toxoplasma. Other pathogens take advantage of immunosuppressed patients but can cause disease in anyone. Examples include herpes simplex virus (HSV), human papilloma virus (HPV), and the bacterium that causes tuberculosis, each of which causes more frequent or severe disease in people with low CD4 counts. As a general rule, “exclusive opportunists” cause disease in people with lower CD4 counts than “optional opportunists.”
In some cases, OIs can be prevented by avoiding exposure to the pathogen itself. For example, we lower the risk of spreading TB by isolating those who are actively infected; you can avoid toxoplasmosis by cooking meat properly; you can avoid syphilis by wearing condoms. However, many opportunistic pathogens are “ubiquitous”—they’re found everywhere and can’t be avoided. Examples include Pneumocystis, MAC, and Cryptococcus. Prevention of infections caused by these organisms requires either prophylaxis(medical treatment that prevents disease) or better yet, ART, which keeps the CD4 count above the danger zone.
Seriously, anybody living with HIV should follow Dr. Joel’s blog. We can’t keep liking these all on our own.