Final answer:
One reason a patient might develop an opportunistic infection is having a compromised immune system, such as in the late stages of AIDS. The yeast Candida sp. can grow unchecked in certain conditions, leading to infections in the vagina or mouth.
Step-by-step explanation:
One of the three reasons a patient might develop an opportunistic infection is having a compromised immune system. Opportunistic infections are mainly seen in individuals whose immune system is weakened, such as patients in the late stages of AIDS. In these cases, the fungi that are normally harmless or part of the natural biota can cause infections and pose a serious threat to the patient's health. For example, the yeast Candida sp. can grow unchecked and cause infections in the vagina or mouth if the pH of the surroundings, the person's immune defenses, or the normal population of bacteria are altered.