Page 25 - AIDSBK23C

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EPIDEMIOLOGY OF AIDS

Considerable epidemiologic and clinical work has been performed to understand the
transmission of HIV from one person to another. As in past epidemics, the spread of AIDS is
facilitated by human travel. Syphilis in the 16th century, bubonic plague in the 17th century, and
influenza early in the 20th century also arose from endemic foci to become widespread. Modern
means of travel by jet aircraft readily available to many people provide an easy route for the
spread of AIDS from one location or population to another.[114]
However, unlike most infections in past epidemics, AIDS is distinguished by a very long
latent period before the development of any visible signs of infection in affected persons. The
average HIV-infected person may have an initial acute self-limited illness, may take up to
several weeks to become seropositive, and then may live up to 8 or 10 years, on average without
treatment, before development of the clinical signs and symptoms of AIDS. In virtually all past
infectious disease epidemics, infected persons were soon easily recognized so that measures
could be taken to prevent the spread of disease. But persons infected with HIV cannot be
recognized by appearance alone, are not prompted to seek medical attention, and are often
unaware that they may be spreading the infection.[29,51,115]
The transmission of HIV is a function of both where the virus appears in the body and
how it is shed. HIV can be present in a variety of body fluids and secretions, as shown in Table
1. The presence of HIV in genital secretions and in blood, and to a lesser extent breast milk, is
significant for spread of HIV. However, the appearance of HIV in saliva, urine, tears, and sweat
is of no major clinical or social importance, as transmission of HIV through these fluids does not
routinely occur, primarily because of the low concentration of HIV in these fluids.[116] Though
infectious particles of HIV are frequent in cerebrospinal fluid, contact with this fluid in daily life
is extremely rare.[22,117]
The most important feature of HIV is the means of spread (Table 2). Unlike most
epidemics of infectious diseases wherein much of a population is at risk, HIV infects definable
population subgroups ("risk groups"). This happens because HIV is primarily a sexually
transmissible disease. Homosexual, bisexual, and heterosexual transmission all can occur.
Although sexual intercourse between males has remained the greatest risk for transmission in
developed nations of Western Europe and the United States, heterosexual transmission has
increased over time, but still remains less common than in Africa, Asia, or parts of the
Caribbean.[115,118,119]
Transmission of HIV can occur from male to male, male to female, and female to male.
Female to female transmission remains extremely rare, though women with same-sex contact are
also often bisexual and have additional risk factors for HIV infection.[120,121] Even a partial
modification of sexual behavior practices may help retard the rate and extent of HIV
transmission. Amongst males having sex with males in the U.S. in the 1990's, the prevalence of
HIV infection remained high at 7.2%, and the prevalence of unprotected anal intercourse over a
prior 6 month period was 41%.[122] In a study from 2008, the prevalence of HIV was 19%
among men having sex with men, and 44% of them were unaware of their infection.[123]
Educational efforts in AIDS prevention must be ongoing and must specifically target not
only persons belonging to identifiable risk groups for HIV infection but also teenagers beginning
sexual intercourse (and who often lack a sense of their own mortality), as well as young adults.
The Centers for Disease Control in the U.S. has a strategic plan to reduce HIV infection through