So, what is Parvo?
(little Bluebell, who pulled through just fine, from the Doberman Assistance Network Facebook page.)
The parvovirus is a virus (yup) that can be transmitted by the vomit or fecal material of an infected dog to another. It can survive in an environment for up to 7 months, something that not everybody is aware of. Areas thought to be infected should be cleaned with a bleach solution (1 part bleach to 30 parts water) in order to actually disinfect.
Puppies, when they receive their shots, are inoculated against parvo in three steps, starting at 8 weeks when they receive the DHPP-1 (Distemper, Hepatitis, Parvovirus, Parainfluenza), which is then boostered at 12 and 16 weeks. There have been reports of vaccinated dogs becoming infected with parvo and dying, but I think that the vaccine protects more dogs than it fails.
Symptoms of parvo include diarrhea, fever, lethargy, and vomiting. If a puppy suffers from vomiting and diarrhea especially, this is not a "wait and see" situation, but a "get that dog to the vet now" situation. Puppies can get dehydrated and go downhill very quickly, and keeping them hydrated is one of the treatment avenues available to dogs who end up surviving parvo.
There is also a "cardiac" form of Parvo (the one I describe above is "intestinal"), though it is less common. Cardiac parvo infects a puppy in the womb or up to 8 weeks of age, attacking the heart muscle and frequently causing sudden death, with few symptoms except maybe breathing difficulty. I guess this happens if the dam is exposed to the virus, but doesn't become infected herself? I'm not really sure. Either way, it's freaking scary.
Parvo is, I think, one of the main reasons it isn't recommended that young unvaccinated puppies be let in areas where there is a lot of dog traffic, and that they shouldn't be exposed to adult "stranger" dogs. If you're unsure of a dog's vaccine history or health, it's probably best to keep your puppy away. Socialization with other dogs is the big deal here (you know, that thing I missed out on with Elka), so it's a good idea to find a puppy kindergarten or puppy playtime in your area, hopefully held in a known to be clean training facility where their interactions can be monitored and you don't have to worry so much about it.