Transmission of HIV infection

HIV-1 and HIV-2, the major and minor human AIDS viruses,
are transmitted in ways that are typical for all retroviruses –
“vertically” – that is from mother to infant, and “horizontally”
through sexual intercourse and through infected blood. The
lymphocytes of a healthy carrier of HIV replicate, and
eliminate, over one billion virions each day and the circulating
virus “load” may exceed ten million virions per millilitre. At
these times viraemia can be recognised by measuring the p24
antigen of HIV in blood and quantifying viral DNA or RNA
(see below). Transmission also depends on other factors,
including the concentration of HIV secreted into body fluids
such as semen, secondary infection of the genital tract, the
efficiency of epithelial barriers, the presence or absence of cells
with receptors for HIV, and perhaps the immune competence of
the exposed person. All infections with HIV appear to become
chronic and many are continuously productive of virus. The
ultimate risk of spread to those repeatedly exposed is therefore
high.
The stage of infection is an important determinant of
infectivity. High titres of virus are reached early in infection,
though this phase is difficult to study because symptoms may be
mild or absent and any anti-HIV response undetectable; it is
nevertheless a time when an individual is likely to infect
contacts. When, much later, the cellular immune response to
HIV begins to fail and AIDS supervenes the individual may
again become highly infectious. In the interval between, there
may be periods when except through massive exposures – for
example blood donation – infected individuals are much less
infectious. Nevertheless, in the absence of reliable markers of
infectivity, all seropositive individuals must be seen as
potentially infectious, even those under successful treatment.
Effective ways are constantly being sought to protect their
contacts and this has led to the development of the concept of
“safe sex”. Ideally, this should inform sexual contact between all
individuals regardless of whether they are known to be infected
with HIV.