An infection by a microorganism that normally does not cause disease but becomes pathogenic when the body's immune system is impaired and unable to fight off infection, as in AIDS and certain other diseases.机会性传染病：一种由微生物传播的疾病，虽然感染者不会得病，但当人体免疫系统被破坏和不能抵抗感染时，就易得此病，例如爱滋病和其它疾病