Any STI or STD that causes visible harm (i.e., sores, warts, blisters) to the genitalia. Though it is named after the slang term for the male reproductive anatomy, dick rot can be applied as a term for an STD/STI carried by a female as well.
John: "Dude, we watched the STD/STI film today in health class. For something that is so gross, it sure does have a clean, tidy name."
Jack: "I know, man. It just didn't make me want to avoid STDs nearly enough. That's why I think of it as dick rot. It just screams, 'This stuff is nasty and is to be avoided.' Way more to the point."