Could the answer be:
Following your riddle:
I am your enemy
Such sickness is definitely your enemy, as it may ultimately kill you.
And I come undercover.
Seropositivity stays undetected until you go and check for it.
Do you see me inside your lover?
If ever you get the HIV through your partner, you may keep seeing the virus through him because of your current state?
Remember, lust is one of the greatest seven.
HIV is transmitted through sexual relations, which can be considered as "lust".
If you knew before, would the situation still worsen?
If the sick person knew about it before, he could have been treated before, and maybe saved. Or if the partner knew, maybe he wouldn't have got the virus in the first place.
Beware when I come,
If you are not, I'll be your trouble.
It might come at a hotel room, or at your own home.
Ignore me, I dare you double!
If you don't protect yourself or ignore the sickness, you are taking the risk of getting HIV. As it can be transmitted through sexual relations, it might happen in a hotel room like in your own home.
About the hints:
The World AIDS Day is on December 1st every year, so it was earlier this month.