Even when a man’s blood viral load is undetectable, he may have intermittently detectable levels of HIV in his semen, with fluctuations occurring in the course of only an hour. This does not necessarily mean, however, that the virus present is capable of transmitting to another person. Analyzing frozen sperm samples from 88 HIV-positive men who had an undetectable viral load and who had provided two samples within an hour if they could, the researchers detected HIV in at least one sample from 17 of the men, comprising 19 percent of the study group. These men produced 23 samples with detectable HIV, or 7.5 percent of all samples. The samples had a median viral load of 705; in 11 of them the read was over 1,000. Twelve out of 129 pairs of samples produced in under an hour, or 9 percent, had undetectable virus in one and detectable virus in the other. null