From the report of a study that finds that having your toothbrush in a bathroom with a toilet does indeed mean there will be some fecal contamination on it:
"The main concern is not with the presence of your own fecal matter on
your toothbrush, but rather when a toothbrush is contaminated with fecal
matter from someone else, which contains bacteria, viruses or parasites
that are not part of your normal flora," said Lauren Aber, MHS
(Graduate Student, Quinnipiac University).