Parvo attacks the rapidly dividing cells of the bone marrow and the small intestine.
Following infection, the virus multiplies in the tonsils before traveling through the blood to the bone marrow and intestinal lining. In the bone marrow, parvo slows the production of white blood cells, the cells responsible for fighting infection. Damage to the intestine prevents normal digestion and allows bacteria normally found there to invade the rest of the body. Both domestic and wild dogs are susceptible to parvovirus infection, but rottweilers, dobermans, and English springer spaniels are reported to be at the highest risk for severe infection. Puppies from 6-16 weeks are most severely affected, but adult dogs may be infected as well.