Unfortunately it is relatively easy for a dog to come into contact with the Canine Parvovirus.
The Dog Parvo can be located in a number of different places, most prevalent of which is canine feces. However, soil can also be infected as can any other surface that has come into contact with an infected dog.
Humans stepping in infected soil or feces and bringing it back to the home is one of the more common reasons for the spread of the parvo virus. Even birds carrying soil or infected items is enough for the virus to spread.
All a dog needs to do is to lick a surface or even smell some feces which puts its nose in contact with the parvo virus. The dog might then be infected.
Parvo can actually live in the environment for a long period of time, weeks, and in the right conditions, months. Thus, it is extremely hard to avoid transmission, if there is an infected animal in your area, or if a family member has been in contact with an infected animal.
However, one of the problems of protecting your dog from contracting parvo, is that many dogs may have the virus, but either not yet be showing signs (it can take a number of days for parvo symptoms to appear), or its health may not be noticeably affected.