HIV infection does not immediately cause AIDS. Without treatment, the average time to development of AIDS following infection of adults is several years. The long delay between infection and development of symptomatic immune deficiency reflects the unusual tropism of the virus for CD4 T cells of the immune system, as well as the nature of the immune response to the virus. HIV is now pandemic, and despite great strides in treatment and prevention that have followed from a greater understanding of the pathogenesis and epidemiology of the disease, 1.6 million people died of AIDSrelated causes in 2012 and an estimated 35.3 million are infected by HIV worldwide, presaging the death of many from AIDS for years to come (Fig. 13.28). In subSaharan Africa, which accounts for over twothirds of the global incidence, 1 in every 20 adults is infected. Indeed, HIV/AIDS has emerged as the most deadly single infectious agent in the short time since its identification as a new human pathogen. Nevertheless, there is cause for optimism: the incidence of new cases of HIV infections worldwide has declined annually since its peak in 1997, and the number of annual deaths from HIV/AIDS has steadily declined since its peak in the mid2000s. Among the regions with the most rapid declines in incidence of new infections is subSaharan Africa. Still, there are focal areas of increasing incidence (for example, Eastern Europe and Central Asia).
13-25 HIV is a retrovirus that establishes a chronic infection that slowly progresses to AIDS.
HIV is an enveloped RNA virus whose structure is shown in Fig. 13.29. Each virus particle, or virion, is decorated with two viral envelope proteins that are used by the virus to infect target cells, and contains two copies of an RNA genome and numerous copies of viral enzymes that are required to establish infection in the cellular host. HIV is an example of a retrovirus, so named because the viral genome must be transcribed from RNA into DNA in the infected cell—the reverse (retro) of the usual pattern of transcription—by the viral reverse transcriptase enzyme. This generates a DNA intermediate that is integrated into the hostcell chromosomes to enable viral replication. RNA transcripts produced from the integrated viral DNA serve both as mRNAs to direct the synthesis of viral proteins and later as the RNA genomes of new viral particles. These escape from the cell by budding from the plasma membrane, each enclosed in a membrane envelope.