First, it seems very difficult (for a virus) to transmit HIV via vaginal intercourse[1]. Second, most infections appear to start from a single virus,[2] despite

First, it seems very difficult (for a virus) to transmit HIV via vaginal intercourse[1]. Second, most infections appear to start from a single virus,[2] despite the huge genetic diversity of intrapatient HIV that supposedly makes it hard to treat[3]. That means most of the strains found within one person never get transmitted. How do you manage to transmit HIV contaminated fluids but only one infection occurs in the receiving person? Very strange. Another strange thing, it does not seem all that difficult (relatively) for interspecies transmission to occur: "Scientists have now documented that the SIV virus has jumped from monkeys or apes into humans at least 13 separate times!"[4] Perhaps those transmissions are due to self-injury while cutting infected meat, etc. The vagina is tissue that expects the presence of foreign human cells, and so has defenses against them. Anal sex, injections, etc are a different story. To me it sounds like an entire cell gets transmitted. [1] http://m.jid.oxfordjournals.org/content/191/9/1403.full http://m.jid.oxfordjournals.org/content/191/9/1403.full [2] http://www.ncbi.nlm.nih.gov/m/pubmed/18490657/ http://www.ncbi.nlm.nih.gov/m/pubmed/18490657/ [3] http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2614444/ http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2614444/ [4] http://evolution.berkeley.edu/evolibrary/news/081101_hivorigins http://evolution.berkeley.edu/evolibrary/news/081101_hivorig... EDIT: Another possibility is that the vast majority of these variants found floating free in the blood are non-functional and serve as chaff to confuse the immune system.