*They* poisoned 30% of Christendom in the Black Death by poisoning Christian wells, Jewish strongholds were untouched, Many Jews confessed. IT was not a virus.
Bubonic plague, aka the Black Death, is not a virus.
Plague is caused by the bacteria Yersinia pestis, a zoonotic bacteria usually found in small mammals and their fleas.
It is not spread through poisoned water.
Plague is transmitted between animals and humans by the bite of infected fleas, direct contact with infected tissues, and inhalation of infected respiratory droplets.
It killed far more than 30% of Christendom.
Plague can be a very severe disease in people, with a case-fatality ratio of 30% to 60% for the bubonic type, and is always fatal for the pneumonic kind when left untreated.
https://www.who.int/news-room/fact-sheets/detail/plague