parvo can be transmitted from place to place on the hair and feet of dogs, as well as on the clothes and shoes of people ,so your friend or anyone who came in to her house could of brought it in on there shoes. also some dogs get it even when they have had there jabs but its rare so if both dogs have it i would say something was wrong with the jabs,go to the vet who done it and complain also if it was a batch of it that thats got something wrong with it there could be other dogs that the owners think are covered and there not so the vet should re call them all. i hope they get better soon.
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