Opportunistic infections (OIs) are infections that occur more often or are more severe in people with weakened immune systems than in people with healthy immune systems. People with weakened immune systems include people living with HIV or people receiving chemotherapy.

Tuberculosis: An infection caused by the bacteria Mycobacterium tuberculosis and Mycobacterium bovis. Tuberculosis (TB) is spread when a person with an active infection (TB disease) coughs, sneezes, speaks, or sings, and then a person nearby breathes in the bacteria. TB usually affects the lungs, but it can also affect other parts of the body, such as the kidneys, spine, and brain.