[YASID] US POWs in ?N. Korea? given virus to make them seriously weak..

probably 1975ish Analog, but could be lots of other times/places.
Scenario is that US POWs are deliberately infected with a virus that
makes them so weak they can barely walk around the prison camp,
let alone pick up any sort of weapon or fight the guards.
Spoiler:  they cook a terrific meal for the guards, which
includes "borscht".  The deep red colour hides the fact
the prisoners added some of their blood to it, so a
few hours later the guards all collapse.
Any recognition?  Thanks.
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