:explain how an individual with a compromised immune system, i. e. hiv, may be unable to fight off and survive infection(s by microorganisms that are usually harmless.

The persons compromised with SCID-like HIV becomes very weak in terms of immunity. They become a host for the opportunistic microbes, which are harmless otherwise. In a normal person, the immune system is strong enough to combat the low percentage of the microbes. BUT in people with SCID, even a limited amount of exposure to the microbes can cause infection. 

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