Final answer:
AIDS, or Acquired Immunodeficiency Syndrome, is the late stage of HIV infection and severely damages the immune system, leading to life-threatening 'opportunistic infections.' It can be fatal without proper treatment, but modern antiretroviral therapy has made HIV manageable.
Step-by-step explanation:
The often fatal disease in which the body's immune system can no longer fight off infections and diseases is AIDS (Acquired Immunodeficiency Syndrome). AIDS is the late stage of HIV (Human Immunodeficiency Virus) infection, which severely damages the immune system. HIV is transmitted through sexual contact, blood transfusion, or from mother to child, among other means, and if left untreated, it leads to AIDS. This condition is characterized by a sharp decline in CD4+ T cells, critical components of the immune response, leaving the body vulnerable to a wide array of 'opportunistic infections'—infections and cancers that generally do not affect people with functioning immune systems.
Initially, AIDS was almost 100% fatal, but the development of antiretroviral therapy has made HIV a chronic, manageable condition. However, AIDS can still be life-threatening without proper treatment, as the compromised immune system of an AIDS patient allows easily avoidable infections and viruses to become dangerous.