Final answer:
Late stage HIV, or AIDS, occurs when a person's CD4 T-cell count falls below 200 cells/µL or when they begin to develop certain opportunistic illnesses, indicating severe immune system damage from HIV.
Step-by-step explanation:
Late stage HIV, known as Acquired Immunodeficiency Syndrome (AIDS), occurs when the CD4 T-cell count in an individual drops below 200 cells/µL or when certain opportunistic illnesses develop due to the immune system being severely damaged by HIV. The progression to AIDS is marked by a range of symptoms including chills, fever, sweats, swollen lymph glands, weakness, and weight loss, alongside the appearance of rare cancers like Kaposi's sarcoma and opportunistic infections like Pneumocystis pneumonia. Without treatment, those diagnosed with AIDS may only survive about 3 years, but with the advent of antiretroviral therapy, the progression of the disease can be significantly delayed, potentially allowing individuals with HIV to reach an average lifespan.