Figure 3. Model of HIV-1 Transmission.

A genetically and phenotypically diverse quasi-species of virus is present in the semen, cervicovaginal secretions, or blood of persons with chronic HIV-1 infection, but most often, only a single virion or virally infected cell is transmitted and leads to productive clinical infection. Other viruses may breach the mucosal or cutaneous surfaces, but they generally do not result in productive infection or contribute to it, presumably because such viruses are defective or less fit or simply fail to come into contact with susceptible target cells. R0 represents the basic reproductive ratio, which corresponds to the number of secondary infections caused by one infected cell. If this number falls below 1, infection is extinguished. In acute infection, the number of productively infected cells and the concentration of free virus in the plasma increase exponentially, with an estimated R0 of 8.³²