It's not an aids risk. It's a hep b/hep c risk, although statistics show that the rate of transmission is (from memory) like 2-5%, and most people that contract it are either extremely unlucky, immunocompromised, or are repeatedly sharing the needles while engaging in unprotected sex work. It just looks scary to the 99% of people that don't use needles, to find that in the trash.
Just leave the cap on, leave the needle on(don't smash it against a table to break it off, I used to do that when id get sick and tired of iv'ing but the needle tip gets lost, I'd comb my carpet for upto 18 hours or more looking for them as well as more dope) put it in a ziplock bag and perhaps write with a permanent marker on the bag about what it is, double bagging/wrap in paper towel. Although who will actually do this... Prolly noone. I know that I didn't ever taken the first precaution, sometimes I would lose the cap and chuck ot in the trash, or find the cap later after id already thrown the rig away. Ahh man. Too many fucked up memories flooding back about this lol