roundworms are contagious to other dogs but they cannot be passed from dog to dog just by being int he same room. However, if the dogs have defecated in the room and there are traces of the stool on the floor there is a chance that the worms can be passed from dog to dog. Roundworms are passed when the eggs of the worms are ingested by another dog, or in certain cases, absorbed through the skin. You must be careful to keep dogs that you suspect have worms away from healthy dogs for this reason, until they are treated.