How a fungus entered the loaf
Ergot is a fungus that infects cereal grains, especially rye. Instead of a healthy kernel, an infected plant can produce a hard, dark growth that is harvested along with the grain. If contaminated grain was milled into flour, the fungus’s toxins could pass into food. Because communities often relied on shared crops and supplies, a bad harvest or contaminated store could expose many people at once—not just one household.
The danger depended on how much contaminated grain people ate. Ergot poisoning was not an inevitable feature of medieval bread, and exposure could vary from loaf to loaf. But ordinary milling and baking did not guarantee that the toxins were made harmless.
What the poison did to the body
Ergot toxins can constrict blood vessels, reducing the flow of blood to the limbs. Severe poisoning could cause intense pain and, in extreme cases, tissue to die and turn black—a form of illness remembered as “St. Anthony’s fire.” Other effects could include muscle spasms, convulsions, confusion, and hallucinations. The symptoms were frightening and varied, making their shared cause difficult to recognize before people understood how infection could travel from crop to food.
That link also explains why ergotism could become a community crisis: the source was not an obvious poison in a single dish, but a contaminated staple eaten repeatedly. A harvest’s hidden defect could reach the flour sack, the kitchen, and eventually many people’s bodies.
