HIV stands for ‚human immunodeficiency virus‘. HIV is a virus (of the type called retrovirus) that infects cells of the human immune system (mainly CD4 positive T cells and macrophages), and destroys or impairs their function. Infection with this virus results in the progressive deterioration of the immune system. Within the retrovirus family, HIV belongs to a subgroup known as lentiviruses, or „slow“ viruses. Lentiviruses are known for having a long time period between initial infection and the beginning of serious symptoms. Similar versions of HIV infect other nonhuman species, such as feline immunodeficiency virus (FIV) in cats and simian immunodeficiency virus (SIV) in monkeys and other nonhuman primates. The immune system is considered deficient when it can no longer fulfill its role of fighting off infections and diseases.

Immunodeficient people are more susceptible to a wide range of infections, most of which are rare among people without immune deficiency. Infections associated with severe immunodeficiency are known as ‚opportunistic infections‘, because they take advantage of a weakened immune system. Some people at the time of seroconversion develop “Acute retroviral syndrome” which is a glandular fever-like illness with fever, rash, joint pains and enlarged lymph nodes. Seroconversion refers to the development of antibodies to HIV and usually takes place between 1 and 6 weeks after HIV infection has happened.

Whether HIV infection causes initial symptoms or not, an HIV infected person is highly infectious during this initial period and can transmit the virus to another person. The only way to determine whether HIV is present in a person's body is by testing for HIV antibodies, DNA or RNA. After HIV has caused progressive deterioration of the immune system, increased susceptibility to infections may lead to symptoms. Primary HIV infection - may be asymptomatic or experienced as Acute retroviral syndrome. Clinical stage 1 - asymptomatic or generalized swelling of the lymph nodes Clinical stage 2 - minor weight loss, mucocutaneous manifestations and recurrent upper respiratory tract infections Clinical stage 3 - includes unexplained chronic diarrhea, unexplained persistent fever, oral candidiasis or leukoplakia, severe bacterial infections, pulmonary tuberculosis, and acute necrotizing inflammation in the mouth. Some people with clinical stage 3 have AIDS. Clinical stage 4 - includes 22 opportunistic infections or cancers related to HIV. All people with clinical stage 4 have AIDS.