I don't know what causes warts, but seem to have more than my share of them. I don't even mess around trying to take care of them by myself anymore.
I go to the doctor and have them treated with liquid nitrogen. This takes care of them and I don't have to worry about them coming back or mess with creams for weeks at a time.
Especially if a get a wart in an obvious place like on my lip, I am making an appointment and getting it taken care of as soon as possible.