What causes smells from multiple toilets in the home?

Lots of people have this problem. The probable cause is the failure of the wax ring that connects the base of the toilet to the large pipe under the floor. The wax rings are supposed to be replaced every few years. Your wax rings have probably disintegrated fairly badly, and it allows odors from the pipe to escape into the room from beneath the toilet when the toilet is not in use (even though you don't have any leaks when you flush). All hardware stores sell wax rings. They have cheap ones for under $3, but I always try to buy the biggest thickest wax ring for a few bucks more. They are just a little bit tricky putting a new one in without messing it up, but it can be done for sure. Good luck.