It's very old, but for some reason it just appeared in my aggregator.. the content isn't dated though.
The probability of HIV transmission may be low, but infection is a serious business -- a terminal illness, a lifetime of taking drugs, dealing with depression and social prejudice. Compare the consequences with the trivial burden involved in preventing the risk: safe sex versus life with a terminal illness -- you don't need a lawyer to tell you which is preferable.
Update: Coincidentally, I've found another post, this one much more recent, about the difference between risk and perceived risk.
I'm now linking to an article which is itself little more than links to some newspaper articles.. argh!
We also tend to overestimate risks that lend themselves to memorable images, like planes crashing into buildings. Like shark attacks and kidnapping by strangers, terrorism is strange, uncontrollable and forms a ready mental image. So people overestimate terrorism's risk and demand excessive protection from it . . . .