Infection occurs when a disease-causing germ such as a bacteria, virus or fungus invades the body. To become infected, you must catch the germ (exposure) and have the ability to become infected (susceptibility). Susceptibility is more complicated than exposure. We are all susceptible to infection by thousands of different germs. The purpose of the immune system is to prevent infection by recognizing germs and eliminating or disabling them before they can cause infection.

People with immunodeficiency get the same kinds of infections that other people get—ear infections, sinusitis and pneumonia. The difference is that their infections occur more frequently, are often more severe, and have a greater risk of complications.