HIV is a human retrovirus that is acquired predominantly through contact of infected body fluids with the bloodstream, a situation similar to the transmission of hepatitis B virus. It mainly infects the CD4 population of lymphocytes, which perform a helper cell function; immunodeficiency develops as the number of CD4 lymphocytes decreases. Cell-mediated immunity is mainly affected, but there is also impairment in the ability to mount new B-cell-mediated responses. As a result, the patient becomes susceptible to infections and neoplasms. Normal individuals usually have approximately 600-800 x 10⁶/mL CD4 lymphocytes. Patients with HIV infection slowly lose their CD4 cells. Opportunistic infections and neoplasms rarely occur until the number of CD4 lymphocytes drops below 300 x 10⁶/mL. Certain infections are not seen until CD4 counts are below 100 x 10⁶/mL. HIV is also known to infect other cell populations such as macrophages, nerve cells and possibly enterocytes, where it may be clinically latent and act as a reservoir of virus.

Persons recognized to be at high risk for acquiring HIV infection include homosexual or bisexual men, intravenous drug users, hemophiliacs and others who received blood or blood products prior to universal testing of blood in approximately 1985. Heterosexuals who have unprotected intercourse with infected partners are also at risk and at present represent the group with the fastest-rising incidence of HIV infection in North America. In Africa, where HIV infection is endemic, heterosexual transmission through unprotected intercourse is the commonest mode of HIV transmission. When seeing patients with suspected HIV infection, it is important to get an accurate history of risk factors including sexual orientation and practices, history of intravenous drug use, past exposure to blood and blood products and travel to endemic areas.

Many physicians find it difficult to discuss sexual orientation and sexual practices with patients. It is often best to ask the patient directly whether he or she is heterosexual, homosexual or bisexual. For male patients the question can also be addressed by asking the patient if he has ever had sexual relations with other men. Many persons may classify themselves as heterosexual but may have had same-sex sexual experiences. Sexual activity and practices should be ascertained by inquiring about the number of sexual partners in the past and whether the patient has had anal intercourse. Unprotected receptive anal intercourse represents the highest-risk sexual practice for HIV transmission. A history of other sexually transmitted diseases is also important as it suggests high-risk activity, and the presence of open lesions during unprotected intercourse may increase the risk of HIV transmission. It is important to address these issues in a clinical and nonjudgmental way, as negatively phrased questions or judgmental attitudes toward sexual orientation and practices can interfere with the doctor-patient relationship. Patients who perceive a judgmental or negative attitude are less likely to discuss these issues truthfully with the physician.

The acquired immunodeficiency syndrome (AIDS) results from infection with the HIV and the resultant immunodeficiency. The diagnosis of AIDS is usually made on the basis of demonstrating positive serology for HIV with the presence of an opportunistic infection, neoplasm or a CD4 lymphocyte count less than 200 x 10⁶/mL. At present it appears that most patients with HIV infection will eventually progress to AIDS; however, the rate of progression is variable. It has been well documented that therapy with antiretroviral drugs slows the progression of HIV infection to AIDS and prolongs the life of patients with established AIDS. The common antiretroviral drugs in use are the nucleoside reverse transcriptase inhibitors zidovudine (AZT), dideoxyinosine (ddI), dideoxycytidine (ddC), stavudine (d4T) and lamivudine (3TC). New drugs showing promise and currently under evaluation include protease inhibitors (siquinavir, ritonivir and indinavir) and the non-nucleoside reverse transcriptase inhibitors. It appears that combination therapy with two or three antiretroviral drugs is better than single-drug therapy and reduces the incidence of drug resistance. Current recommendations for antiretroviral therapy are that therapy with AZT alone or in combination should be started once the CD4 lymphocyte count falls below 500 x 10⁶/mL. The best combination of antiretroviral drugs has yet to be determined. With advancing immunosuppression, the common occurrence of certain opportunistic infections such as Pneumocystis carinii pneumonia, Mycobacterium avium-intracellulare and toxoplasmosis has also prompted recommendations for prophylactic therapy.