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. Thelymphocytes of a healthy carrier of HIV replicate, andeliminate, over one billion virions each day and the circulatingvirus “load” may exceed ten million virions per millilitre. Atthese times viraemia can be recognised by measuring the p24antigen 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 fluidssuch as semen, secondary infection of the genital tract, theefficiency of epithelial barriers, the presence or absence of cellswith receptors for HIV, and perhaps the immune competence ofthe exposed person. All infections with HIV appear to becomechronic and many are continuously productive of virus. Theultimate risk of spread to those repeatedly exposed is thereforehigh.The stage of infection is an important determinant ofinfectivity. High titres of virus are reached early in infection,though this phase is difficult to study because symptoms may bemild or absent and any anti-HIV response undetectable; it isnevertheless a time when an individual is likely to infectcontacts. When, much later, the cellular immune response toHIV begins to fail and AIDS supervenes the individual mayagain become highly infectious. In the interval between, theremay be periods when except through massive exposures – forexample blood donation – infected individuals are much lessinfectious. Nevertheless, in the absence of reliable markers ofinfectivity, all seropositive individuals must be seen aspotentially infectious, even those under successful treatment.Effective ways are constantly being sought to protect theircontacts and this has led to the development of the concept of“safe sex”. Ideally, this should inform sexual contact between allindividuals regardless of whether they are known to be infectedwith HIV.
