EPIDEMIOLOGY

AIDS was recognized as a clinical entity in 1981, and within the next
few years important advances were made in the understanding of its
epidemiology, cause, and natural history. HIV-1 is responsible for
most of the AIDS cases reported throughout the world; HIV-2 is
prevalent in western Africa and parts of Asia, including India, and is
reported to be less pathogenic. HIV-1 is broken down to ten subtypes:
A–J and O. The latter (the “O” stands for “outlier” and is considered to
be the primitive form of HIV-1) is reported to be responsible for nine
cases of AIDS that occurred in Cameroon in 1993 (P. Zekeng, personal
communication, 1994). These subtypes are unevenly distributed
throughout the world. For instance, subtype B is mostly found in the
Americas, Japan, Australia, the Caribbean, and Europe; subtypes A and
D predominate in sub-Saharan Africa; subtype C in South Africa and India;
and subtype E in Central African Republic, Thailand, and other countries of
Southeast Asia, Laboratory studies have demonstrated that subtypes C and
E infect and replicate more efficiently than subtype B in Langerhans' cells present
in the vaginal mucosa, cervix, and the foreskin of the penis but not on the
wall of the rectum. It is likely that HIV subtypes C and E have a higher
potential for heterosexual transmission than subtype B. However, other variables
that affect the risk of transmission, such as the stage of HIV
disease, the frequency of exposure, condom use, and the presence of
other sexually transmitted diseases (STDs), are equally important.