While in Canada the animals will sometimes use an attack that gives you Dire AIDS, when they use this attack don't block it (I got my AIDS from a snake). If you contract Dire AIDS the game will tell you that you have it. Don't cure it until after you defeat Princess Kenny at the end of the game.
Be warned that having Dire AIDS damages you a bit in battle so it would be wise to have a lot of health potions when you enter a difficult battle. Once you get this trophy and you decide you no longer want Dire AIDS you can get them cured in Ottawa by going to the hospital.