HIV is believed to have entered human populations from chimpanzees in Africa in the first half of the 20th century. Comparisons with closely related viruses in chimpanzees show that this crossover was likely made on multiple occasions (Korber et al., 2000; Hahn et al., 2000). In the United States, there is evidence the virus was present as early as 1977, though its long latency period rendered it invisible (Gottlieb, 2001).

The course of HIV disease varies by individual and is not fully predictable, and the full effects of current antiretroviral management on the natural history of the disease remain uncertain. Still, the scientific and medical communities have learned a great deal in the past 20 years, and the natural history of HIV infection is now better understood (Polk et al., 1987; Mellors et al., 1996, 1997; Vlahov et al., 1998; Pezzotti et al., 1999). This understanding is important to the care system because it indicates when care will be needed and which services will be necessary and appropriate at each point in the disease process.

Initial/Primary HIV Infection

The majority of newly infected individuals develop what is known as primary HIV infection or acute retroviral syndrome. Acutely infected persons are symptomatic, often sufficiently so to seek medical care. These symptoms—fever, rash, fatigue, generalized lymphadenopathy, and nausea among others—are flu-like and appear within days to several weeks of the moment of infection. Primary HIV infection usually resolves in a matter of weeks and is not life threatening. Because the symptoms are characteristic of infection by less serious viruses, the opportunity to identify HIV infection is often missed at this stage (Quinn, 1997; Kahn and Walker, 1998).

During the symptomatic phase of acute infection, virus replication is unchecked by the immune system. Individuals in this disease stage are highly infectious. This is of great importance from a public health perspective because unsafe behavior in this phase may readily lead to transmission. It is estimated that more than half of all HIV infections may be transmitted during this stage of infection (Schacker et al., 1998). Therefore, increasing the identification of HIV during this silent phase of the disease and providing prevention counseling to infected individuals are key strategies for managing the progression of the epidemic.

Asymptomatic HIV Infection

After full antibody reaction to HIV infection is established (typically within three to six months), the infection is said to be in the “chronic” or

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