Pregnancy Risk from Contact with Semen: What You Need to Know

Question
If I wiped my ejaculate onto a tissue and threw it away, and someone touched it, is it possible for them to be pregnant? I know it only takes a single sperm to fertilize the egg, but I also know that it takes a decent amount of sperm to survive long enough in a hostile environment to reach the egg and penetrate it. I also know that small amounts of semen usually dry and die quickly exposed to air.
I know that they would have to touch down there quickly after coming in contact but what are the chances of a tiny amount of semen impregnating someone?  

Answer
Sperm are very fragile and die pretty quickly once they are not in a liquid medium. I doubt if the sperm on the tissue remained viable for more than a second or two. If the sperm reaches the vagina (or the vulva) and the area is moist, one sperm can, indeed, swim into the vagina and through the cervix and up the fallopian tube and fertilize an egg. However, it is doubtful that one sperm can do that other than theoretically.