Most reputable humane societies would not put a litter out for foster unless the litter had been with them for awhile and they had been effectively quarantined.
However, that would definitely need to be checked out very throughly. My wife & I once did an "emergency" foster on short notice for two pups (about 4 - 5 weeks old) & it turned out that they had parvo & did not survive. It was very sad and heartbreaking, but at least those two pups experienced a week or so of a loving home.
In the case of the pups we had, supposedly they were with Animal Humane for about a week & were thought to be healthy when we took them. So I would modify your suggestion to make sure that the quarantine period was at least two weeks before I would allow them to intermingle with a young pre-parvo shot pup.
So all in all, once again, you come up with a great point.
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