AIDS is not one disease, but rather a final expression of a continuum of clinical manifestations of HIV. There are important clinical differences between an HIV-positive patient with no clinical signs and one with AIDS.

Pathophysiology of HIV

HIV is a retrovirus that is subdivided into two serotypes, HIV-1 and HIV-2. HIV-1 is by far the most prevalent cause of HIV infection worldwide. HIV selectively attacks a group of lympho- cytes within the cellular division of the immune system, known as T4 lymphocytes.

Once the viral cells are introduced into the host, the virus begins immediate, widespread dissemination to lymphatic systems and organs. Infectivity is high during primary infection, with high plasma viremia detected in blood and the presence of the virus in sexual organs and bodily secretions. In most patients, a new, steady-state plasma HIV RNA "viral set point" is established approxi-mately six months or longer after the initial infection. This homeostatic balance can remain stable for months or years before a patient progresses to develop AIDS. The time course is highly variable. The average duration is 10 to 11 years in the absence of antiretroviral therapy.

Transmission of disease

The predominant mode of HIV transmission is sexual. The virus is spread through mucus membrane contact with semen and vaginal secretions. This form of HIV accounts for more than 70% of all cases of infection.

Blood transmission can occur by transfusion of contaminated blood, needle sharing during drug use or needle stick injuries. In terms of transmission through blood transfusions, the incidence of HIV contamination decreased dramatically when HIV antibody testing and the HIV p24 antigen became available to screen donors. Overall, a very small percentage of people are infected this way.

Course of disease

Many clinicians consider there to be three stages of HIV infection: initial, chronic and final (or crisis) stage. The early phase occurs when a person is newly infected with the virus. Many times there are few if any clinical symptoms. At this stage, a patient is considered HIV positive, but does not have AIDS.

The next phase is the chronic phase, which often lasts many years. During this phase, most patients feel relatively healthy even though the virus is slowly spreading through their body. The goal of modern-day management of HIV is to prolong this chronic stage to delay the onset of AIDS.

The last stage, the crisis stage, is the period in which we consider patients to have AIDS. This phase may last months or years depending on the health and treatment plan. Many patients do not feel well and have significant and life-threatening clinical manifestations. Eventually, these patients die as a result of diseases that their bodies can no longer fight and medical therapies can no longer control.

Preventing HIV infection

There is much established data to confirm the effectiveness of condoms in preventing the spread of HIV. An alternative is a female condom, which has also proven effective in decreasing the risks of HIV transmission.

Another major component of decreasing the incidence of HIV transmission via drug injection is to counsel drug users to stop using drugs and enter a substance abuse treatment program. For those who cannot cease drug use, the CDC emphasizes the importance of not sharing needles and the need to use syringes from a reliable source.

Health care workers

As primary care providers, we should have a working knowledge of procedures and guidelines relating to post exposure prophylaxis (PEP) for health care workers. The most important concept for health care workers deals with "universal precau-tions." The premise is that health care providers examine patients who have not yet been diagnosed with HIV/AIDS or who don't know or disclose that fact.

The most recent U.S. Public Heath Service Guidelines for health care personnel who have occupational exposure to blood and other body fluids that might contain hepatitis B virus (HBV), hepatitis C virus (HCV) or HIV are available at http://aidsinfo.nih.gov/guidelines/healthcare/HC_062901.html.

Diagnosing HIV

The FDA has approved several HIV tests for use in the United States. In many cases, the clinician orders a combination of tests to reach a definitive diagnosis. The various tests utilize different body fluids (whole blood, serum, plasma, oral fluid and urine) with various specificity. They include enzyme immunoassay (EIA); Western blot, oral mucosal transudate Western blot; polymerase chain reaction (PCR); HIV p24 antigen; immunofluorescence assay (IFA).

Management

One of the key aspects of management is to educate the patient that there is no cure for HIV or AIDS. The cornerstones of therapy are lifestyle changes and intensive medical therapy.

The FDA has approved four major classes of drugs and 20 antiretroviral agents for the management of HIV-1 infection and AIDS. (See "AIDS Medications," below.) In many cases, patients will receive various combina-tions of these agents in potent doses, a regimen often referred to as highly active antiretroviral therapy (HAART).

Laboratory monitoring

For many years, the approach to managing HIV disease was limited to monitoring only CD4+ cell counts as a means of assessing immunologic function. One of the more dramatic improvements is the development of new molecular methods that allow quantification of HIV-1 RNA in plasma. Thus, the extent of viral replication, termed "viral load," can be quantified in terms of the number of HIV RNA copies per mL of blood. The FDA has approved three HIV viral load assays for determining prognosis and monitoring response to therapy. By monitoring the clinical condition of the patient along with the viral load and CD4+ cell count, clinicians can more accurately monitor a patient's condition and make modifications to the medical regimen, if necessary.