In the human body there are many germs such as bacteria, protozoa, fungi and viruses. When the human immune system is working well, it is able to control these germs. However, when the immune system is weakened or compromised, these germs may get out of control and cause health problems.

Opportunistic infections are mild to severe infections caused by microorganisms (pathogens), which can include bacteria (such as staph), viruses (such as HIV), and fungi (such as yeast). While these microorganisms usually do not cause serious disease in healthy individuals, individuals with compromised immune systems (such as individuals infected with HIV) will be less likely to resist such opportunistic infections.

Symptoms of opportunistic infections will vary according to the microorganism that caused the infection and by the individual infected. Treatment or medical management of opportunistic infections may be difficult because some of these microorganisms may be resistant to standard antibiotic therapy. For example, since antibiotics target bacterial structure and function, and since viruses and bacteria are structurally different, antibiotics will have no effect on viruses.