Progression to clinical disease

In untreated HIV-infected individuals, the average loss of CD4 T-cells in blood is about 60 cells/mm³ per year, but there is wide variance in this number. Loss of CD4 T-cells and the resultant abnormalities mainly affect acquired immunity and cell-mediated responses. Newly encountered antigens do not elicit much of an immune response, and loss of memory T-cells removes the efficiency of secondary responses to recurring antigens. This immunodeficiency makes people more susceptible to disease from viruses, intracellular bacteria, fungi, and some protozoa.

Humoral immunity is not so badly affected if memory B-cells for common antigens exist. Most adults have a large population of B-cells that are good at responding to common bacteria that have been seen before. Children do not have the same immune memory so they have problems with a wider range of bacterial infections than adults do. Lack of cytokine production also affects the function of most immune cells, including the phagocytic cells (macrophages and neutrophils).

In progressive immune deficiency, a group of relatively minor conditions appear that cause problems with weight loss, non-specific diarrhoea, oral fungal infection (candidiasis or thrush), and various skin conditions. These are not AIDS-defining illnesses. AIDS is a more serious group of potentially life-threatening secondary infections and two types of cancers, both of which may be caused by viral infections. The manifestations of AIDS depend on the type of infection and the part of the body affected. Many of these conditions respond to medical treatment. Some, such as Pneumocystis pneumonia, may resolve only to reappear later on.