I recently visited a very old abandoned building that was probably rotting with asbestos (or however it's spelled.) I didn't feel very good after going inside for less than a half hour. Is it harmless?

You wouldn't feel the effects of asbestos right away. You could feel the effects of dust in general, but mesotheleoma takes years to show up.

I wouldn't worry...If you didn't disturb or touch it ,,it will be ok..I live in a very old house and it's basement pipes are covered with that stuff..I am down there alot and I was told as long as I don't disturb it ..its fine..but if it dust cuz of bumping it or breaking it ,,it can get in your lungs and cause problems..You might be sensitive to mold or mildew in home,,,But if it persists I would go to the Dr's .GOOD LUCK!