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The human immunodeficiency virus (HIV or HIV-1) is a retrovirus that causes the acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the immune system leads to life-threatening opportunistic infections. A second retrovirus, HIV-2, also is recognized to cause AIDS, although it is less virulent, transmissible, and prevalent than HIV-1. Modern antiretroviral regimens have decreased the morbidity and mortality of HIV; however, HIV infection cannot be cured due to the integration of the HIV genome into host cells, creating a latent reservoir. An AIDS diagnosis is made when the presence of HIV is laboratory-confirmed and the cluster of differentiation 4 (CD4 cell) count drops below 200 cells/mm3 (200 × 106/L) for those older than or equal to 6 years of age or after the development of an opportunistic infection.

Infection with HIV occurs through three primary modes: sexual, parenteral, and perinatal. Sexual intercourse, primarily anal and vaginal intercourse, is the most common method for transmission. The probability of HIV transmission depends upon the type of sexual exposure and the highest risk is from receptive anorectal intercourse. Transmission risk is lower for receptive vaginal intercourse, and insertive sex acts have lower risk than receptive acts. Condom use reduces risk of transmission by approximately 80%. The viral load in the index partner is an additional risk factor for transmission. For example, transmission is higher when the index partner has early or late HIV compared with asymptomatic HIV, as these disease stages are associated with higher viral loads. Individuals with genital ulcers or sexually transmitted diseases (STDs) are at greater risk for contracting HIV. Parenteral transmission of HIV broadly encompasses infections due to infected blood exposure from needle sticks, intravenous (IV) injection with used needles, receipt of blood products, and organ transplants. Use of contaminated needles has been the main cause of parenteral transmissions. Blood and tissue products in the healthcare system are now rigorously screened for HIV and the risk for receiving tainted blood or blood products is very low. Healthcare workers have a small but definite occupational risk of contracting HIV through accidental exposure. Most cases of occupationally acquired HIV have been the result of a percutaneous needle stick injury. Perinatal infection, or vertical transmission, is the most common cause of pediatric HIV infection. Most infections occur during or near to the time of birth, although a fraction can occur in utero. Casual contact with patients with HIV is not a risk factor for transmission.

The life cycle of HIV is illustrated in Figure 27-1. HIV has an outer glycoprotein (gp160) on its surface and is composed of two subunits (gp120 and gp41). The glycoprotein has affinity for CD4 receptors and the gp120 subunit is responsible for CD4 binding after exposure. Once initial binding occurs, the association of HIV with the cell is enhanced by additional binding to chemokine coreceptors. The two major chemokine receptors are chemokine receptor ...

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