There is a greater incidence of parvo and other infectious diseases in pups that have not been given any vaccines compared to those that have been following a traditional vaccine schedule.
Any reputable breeder or rescue would vaccinate this dog prior to rehoming, so I am guessing (okay judging a bit) that this is some sort of private rehoming, Craigslist or marketplace situation? Without further details, I can fabricate all sorts of red flags about this.
Any puppy can get parvo, even if appropriately vaccinated, it’s just that if the puppies aren’t vaccinated, it’s fairly common that mom also wasn’t vaccinated and that cohort tends to carry a greater risk for a variety of reasons. Just recently saw a parvo puppy that came from a reputable breeder and a responsible owner. It happens. I harp on parvo because the average cost of treatment in my area ranges from 6-10k, but there are other reasons to be suspicious of this situation as well. Need more information. There’s plenty of dogs and cats that have made it from the streets or without preventative medicine without incident, so it’s not to say you’re guaranteed to have a lemon, but you’re right to be skeptical.