Bruce
Heres a Link that provides a few ideas for getting rid of them when they become a nuisance. The most likely concern is the guano. It could contain a fungal spores that if breathed in to the lungs can cause an infection called Histoplasmosis. It's unlikely but possible. Rabies is even less likely, but again possible.

We've lots of bats in Alberta and I have little concern about them. On the other hand Hantavirus from rodents, lyme disease from deer ticks and west nile virus from Culex sp. do concern me. It seems you can't go anywhere these days without something getting you.