Cell-free vs cell-associated HIV transmission
Richard Jefferys, TAG
One of the most difficult questions to address regarding HIV transmission is whether virus floating free in plasma (cell-free virus) or contained inside cells (cell-associated virus) plays the primary role in causing infection. The issue has important ramifications for the design of biomedical prevention interventions, because cell-associated HIV may be less susceptible to some approaches than cell-free HIV.
Back in February a new journal called Science Translational Medicine published a paper that takes a detailed look at the question in six gay male transmission pairs. 
Although the sample is very small, the results show that virus found free in seminal plasma of the transmitting partner consistently bore the closest resemblance to the virus found in the newly infected individual. The authors acknowledge that larger studies are needed but also note that currently these results provide the most compelling experimental confirmation for the hypothesis that that cell-free HIV RNA in seminal plasma, and not cell-associated HIV DNA in seminal cells, is the origin of sexually transmitted virus between MSM.
Source: TAG basic science blog. Cell-free vs. cell-associated HIV transmission. (15 Apr 2010).
1. Butler DM et al. The origins of sexually transmitted HIV among men who have sex with men. Sci Transl Med 10 February 2010: Vol. 2, Issue 18, p. 18re1. DOI: 10.1126/scitranslmed.3000447.
The full text of articles in Science Translational Medicine can be accessed by signing up for a free two week trial.