As Dr. Doug said Parvo can spread through your walking in an contaminated area and bringing it to your pup. Having been involved in rescue I found there is no bullet proof way to avoid Parvo. It can spread through the air through A/C vents in a building. We were shut down by the State of Georgia until we could clean the facility and then we had to close part of the shelter for six months to make sure we were clean. One of our foster homes was put off limits for a year because of a stubborn Parvo outbreak we couldn't seem to get rid of.
Vaccinations are the only way to deal with this deadly illness and even then you just don't know. Listen to Dr. Doug as he has many years dealing with this disease and most likely has seen just about all there is to see with this troublesome illness. Sometimes it is best not to travel with young pups until they have all of their shots and their immune system has a chance to develop.