It's a short novella, I think, something Scandinavian (i.e. Swedish, Danish, or something like that, but I'm not really sure) from 1980s (or maybe even 1970s) Russian translation.
There's an outbreak of a disease where people become extremely violent in the affected area. Some (few?) are unaffected, but must run from the infected who attack them.
A narrator goes into the area (is sent into it, to investigate?). After some running about he meets doctors, who turn out to be the worst -- they pretend to help people but in fact put them to sleep and take all their blood out.
Turns out the disease makes human blood clot inside the veins, so they do transfusions to themselves every few hours and must have new subjects all the time.
In the story's end it is revealed that the outbreak was actually started by a governmental experiment by accident, when some infected postal stamps were sold in certain areas. The idea being that people would moisten the stamps with their tongues before putting the stamps on envelopes, and that was the way the agent was introduced into their bloodstream which was thought to be -- don't remember, but say, beneficial in some way even, maybe.