They may suffer serious infections from microorganisms that are harmless in robust individuals. Opportunistic infection commonly occurs with microorganisms that are part of normal microflora because these are either already present in the body or frequently acquired from other people. Opportunistic infections may also arise from microorganisms that are not normal microflora, but exist in the environment. For example, the fungus Aspergillus is common decaying vegetation and does not normally infect humans. People who are immunocompromised may acquire Aspergillus infections.
Hospitals are full of sick people, many of whom are compromised hosts.
These people often acquire opportunistic infections from hospital personnel or medical equipment. When infection is acquired in this way, it is said to be a nosocomial infection. Most nosocomial infections are opportunistic infections but non opportunistic infections can also be acquired nosocomially. For example, if a doctor who is caring for a patient with influenza transfers influenza virus from the sick patient to a patient who is being treated for a broken arm, the new influenza infection is nosocomial, but not opportunistic. You don't need to be a compromised host to suffer from influenza.