Okay, so this isn't a critique of last night's new episode of South Park, really, since I don't hold the show to a particularly high scientific standard. And the show was pretty hilarious social commentary. Good stuff. But it's worth noting a few pretty glaring myths perpetuated in last night's episode in which Cartman gets HIV through a blood transfusion:
1) People don't really get HIV through blood transfusions anymore in the US. It just... doesn't really happen. Back in the 80s, yes (in fact, HIV-tainted blood transfusions wiped out huge proportions of hemophiliacs worldwide).
2) Post-exposure prophylaxis would have most likely been able to prevent Cartman from becoming infected.
3) Neither Cartman or Kyle would have tested positive so quickly. It takes usually about 3 months (the "window period") for your body to produce antibodies that would show up in a test.
4) Cartman infects Kyle with HIV by squirting some of his blood into Kyle's mouth. This is highly unlikely to infect someone. Particularly because Cartman became infected the day before. It's possible, but not probable.
Okay so taking the show so seriously is ridiculous, I know. Hah! But a little HIV 101 is useful every now and then! :)
These things bothered me too (but hey, this is the show that had stem-cells grow into a Shakey's).
I will comment that antibody tests aren't the only kind around now (though even PCR-based ones wouldn't really have any chance that fast).
This CAPTCHA system is kind of a hard one to read, by the way.
But if they wanted to know as soon as possible, a PCR-based test would be the way to go, albeit expensive.