Do I have to worry about catching any diseases if I licked a girl's anus?

Depends. Consider the location & it's normal inhabitants. The poo passes through the area, so contamination with e.Coli, yeast & a variety of other germs have been given a clear path to your throat. I occasionally see group a strep produce a rash in the area, so add that to the mix. Herpes simplex virus as well as others of the std ilk may or may not be around.Worry? Yes or no, all life's choices have risks.