Veterinary & Aquatic Services Department, Drs. Foster & Smith
"Canine parvovirus (CPV) disease is currently the most common infectious disorder of dogs in the United States.
'Parvo' is a highly contagious disease characterized by diarrhea that is often bloody. Prior to 1980, most canine parvovirus that caused disease was Type 2 (CPV-2). After 1980, CPV-2 was replaced by CPV-2a became more common and in 1986, another variation called CPV-2b appeared. In the past few years, a new strain, CPV-2c has been detected. Today, CPV-2b has largely replaced the previous strains as the most common parvovirus causing disease in the dog. There is currently some discussion that there may be other strains that are beginning to emerge and have yet to be formally identified. Current vaccinations have helped to control the spread of this disease but despite being vaccinated, some dogs still contract and die from parvo.
...
Parvovirus is spread through contact with feces containing the virus.
...
In summary, parvovirus is a very common problem that is a huge killer of puppies."
k-state.edu article
"If you're planning a trip to the dog park or a walk around a crowded park with your new puppy, you might want to wait a few weeks. A Kansas State University veterinarian says taking your puppy out before it has been fully vaccinated for parvovirus could be deadly, especially at this time of year.
...
The increased number of cases is because the weather is nicer, which is a better environment for the virus to survive, and owners are taking advantage of the warmer temperatures to take their pets outside and interact with other dogs. But owners may not realize their puppy is at risk of getting parvo until they are fully vaccinated, according to Nelson.
...
Parvovirus is most prevalent in dogs 6 weeks to 6 months of age and can be picked up through contaminated feces, tracked in on shoes and through sharing harnesses, collars or food and water bowls."
Just a couple of examples after fifteen seconds of Googling.