Parvovirus, as its name implies, is a virus that attacks rapidly dividing cells in dogs. Two such areas where cells like this occur in the body are the gastrointestinal tract and bone marrow. The virus attacking these areas leads to severe diarrhea, vomiting, and a decrease in white blood cells (which usually help to fight off infection). Without treatment a majority of dogs will die from dehydration and infection. Even with treatment, some animals may not survive. It hits puppies particularly hard as their immune system is not fully developed and their ability to fight infection is low. Un-vaccinated adults can also become infected, but may respond to treatment better since their immune system is usually fully developed.
The virus is picked up when a dog ingest infected soil or other contaminated materials. It is a very hearty virus and can live in the environment (even harsh conditions) for over a year, so may be prevalent in areas of heavy dog traffic. After the virus is ingested it replicates in the lymph nodes and then escapes into the bloodstream eventually ending up in the intestinal tract and bone marrow. Infected dogs will typically have acute cases of vomiting, diarrhea, inappetence, and weakness. In many cases, blood will be seen in the diarrhea or even the vomit. Most vets have a good bed-side test for parvo that uses a rectal swab and gives a result in less than 10 minutes.
Because it is a virus, there is not much we can do besides provide supportive care to the pet while the virus runs its course. Supportive care usually involves hospitalization with fluids, antibiotics, vitamins, nutritional support, and pain management. If finances allow and it is available, some veterinarians will also treat with plasma that is rich in proteins and electrolytes. In addition, some vets are using an anti-viral medication called Tamiflu (used against human influenza cases) to help treat parvo cases. The medication does nothing against the parvovirus, but likely helps prevent the secondary infections that are seen with the cases. Unfortunately, even with the most aggressive supportive care some dogs may not survive the infection.
The good news is there is a good vaccine to help protect against the virus. Vaccinations typically start around 6 weeks of age and are given every 3-4 weeks until 16 weeks. Then, the vaccine is boosted at a year of age. The parvo vaccine is typically part of the "distemper vaccine" series which includes multiple viruses. Prior to 6 weeks puppies have some protection if they were able to ingest enough colostrum (mother's milk for the first 24 hours).
So the moral of the story is to make sure that your pet starts vaccinations at 6 weeks of age to prevent this aggressive disease.