It happened during the richest period of the Medieval Age. Cities were filled with squalor and filth. Jews ran commerce rings and poisoned wells with dead bodies. Rural people around towns also got sick because of commerce routes. Only the isolated communities got it less, but coastal cities got hit the hardest, and the more inland you went, the less the disease hit. Poland had too many land routes and not as much naval commerce (with the exception of the only place where there was almost no plague; Scandinavia). Jews being on good graces recently may have aided, but I doubt it.