My theory: who infected Tom?
This contains spoilers, so be forewarned. After watching the movie, it suddenly dawned on me who must have infected Tom.
If you interpret the ending to mean that Dan is HIV+ (and I do), then you must also take as fact that Dan really did not know he was infected. Several times he debates that he's healthy, and this is the key bit of info. If Dan had infected Tom with the disease, then he would have had to be HIV+ when the incident occurred five years ago. If Dan had never been tested and never seen any physical indication that he had the virus since that time, and he's positive now, then he must have become infected fairly recently, as signs of the illness would certainly have made themselves known to an infected person not taking any medication over a five-year period (Does Tom have symptoms? He should have or have had, but he never reveals any nor does he take off his clothes to reveal any and he never mentions taking meds).
So my theory is that Dan was HIV- when he had sex with Tom five years prior, which means that Tom got the AIDS virus from his wife, who apparently got it from having an affair with someone who had HIV.
His guilt over having a pleasurable gay experience added to the denial that his wife might have cheated on him, causing him to take out his anger, frustration and guilt on the only other person he felt he could hold responsible, Dan, no matter how improbable. And one more thing: Tom told Dan he remembers that night in detail, but he doesn't seem to recollect if Dan wore a condom, either.
Does this make sense or have I missed something?