Canine parvovirus lives in the gastrointestinal tracts of sick dogs, so it is spread by contact with infected feces or vomit. It is highly contagious and can live in the environment for long periods of time, so sometimes puppies can contract it even if the exposure is not obvious.
Parvo can be spread by human handlers who do not wash properly after handling infected dogs or items. It can also stay behind on streets, floors, bowls or beds if proper disinfection protocols are not used. Many puppies contract it from walking on public streets or going to dog parks, or from the floors of high-traffic pet stores. Puppies should stay away from these areas until they have been completely vaccinated.