The main reasons why dogs can get parvovirus are as follows:

1. Contact transmission: Canine parvovirus is a highly contagious infectious disease in dogs. Dogs carrying the virus are the main source of infection. The vomit, feces, and urine of sick dogs contain large amounts of viruses.

2. Pregnancy infection: If a pregnant female dog is infected with CPV-2 (parvovirus type II) in early pregnancy, the fetus in the uterus or puppies less than 2 weeks old may also be infected.

3. Improper diet: Improper daily diet is a common cause of parvovirus. For example, dogs eating cold food or eating irregularly may cause their immunity to decline, making them more susceptible to viruses.

4. Improper use of vaccines: If the dog is vaccinated when the dog is weak and sick, or if there is a problem with the quality of the vaccine itself, it may also cause the dog to be infected with parvovirus.

Therefore, in order to prevent dogs from being infected with parvovirus, it is recommended to start from many aspects. Once symptoms of infection are found in a dog, treatment should be given immediately.