Diarrhea can be mild or severe fatal. Diarrhea in an otherwise bright, alert, eating, drinking dog is more likely due to diet change, stress, parasites, or diet change, than to parvovirus.
The virus is very contagious and is spread by exposure to feces. Unfortunately, dogs from shelters often have been exposed to parvovirus and should be observed for 14 days after adoption to be sure they are not incubating the virus.
There is some risk that a dog incubating parvovirus will infect other dogs. The new dog and its feces should be kept away from puppies and unvaccinated dogs for 2 weeks. Dogs that have had at least two vaccines, with the last one at least 2 weeks prior to exposure to an infected dog are fairly protected.
If you suspect your dog has parvo, call your vet as soon as possible. The earlier the infection is diagnosed and treated with fluids, antibiotics, and nursing care, the more likely it is to do well.