What term is used for diseases that occur when the immune system is weakened?

Opportunistic diseases refer to infections or diseases that take advantage of a weakened immune system. When an individual's immune defenses are compromised, whether by a medical condition, certain treatments, or other factors, pathogens such as bacteria, viruses, fungi, or parasites that would normally be contained by a healthy immune system can cause illness. These diseases are often seen in individuals with conditions such as HIV/AIDS, cancer patients undergoing chemotherapy, or people on immunosuppressive drugs. The term emphasizes the 'opportunistic' nature of these pathogens, as they exploit the host's vulnerability to establish an infection.

Multiple sclerosis refers to an autoimmune condition affecting the central nervous system rather than a term related to opportunistic infections. Huntington's disease is a genetic disorder that leads to the progressive degeneration of brain cells, and does not directly relate to immune system function. Apraxia is a motor disorder that affects the ability to perform purposeful movements and is also unrelated to the context of immune system weakness or opportunistic infections. Therefore, opportunistic diseases is the appropriate term to describe diseases that manifest when the immune system is compromised.