HIV doesn't live outside the human body. It would be impossible for Stella to contract it that way.
Det. Taylor holds up the victim's credit card and refers to it as a 'universal key'. He previously held up the card to the lock and it is clearly a dead bolt. The entire purpose of a dead bolt is to prevent 'shimming' opening in this manner. The only way the victim could have entered with a credit cards is if the door was unlocked to begin with.