It's AIDS? (maybe Spoiler)
I don't know, but the first thing came into my mind after see Tommy look like in Mexico, absolutely it's AIDS.
However, there is no word "AIDS" come out in whole movie, just word "sick". But, there is one strong prove, in the middle of the trip, when Allan want to kiss Tommy in mouth, instead Tommy avoid it and give a kiss to Allan's forehead.
What do you think?