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.
Figure 2.2 HIV particles, many showing typical lentivirus morphology
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