The most important reason for low CD4 is infection with HIV virus, because HIV virus mainly targets CD4-T lymphocytes and harms the body’s immune system, resulting in loss of immune function, which can easily lead to a variety of opportunistic infections and a variety of comorbidities. As soon as you notice a decrease in CD4, you should be tested for HIV, which can start with an initial screening test. If the initial screening is positive, then further confirmatory tests will be done. More than 90% of the patients with a positive initial screening will have a positive confirmatory test. If the diagnosis of AIDS patients need to apply antiviral drugs, early application of antiviral drugs, can extend the incubation period, thereby extending life. The incubation period of AIDS is usually very long, and can be prolonged if effective antiviral drugs are used, because there may not be any clinical symptoms or manifestations during the incubation period.