Dysentery means having trouble with your insides - it usually means that you have very serious diarrhea. In the ancient and medieval world, dysentery killed about a third of all the babies born before they got to be a year old. They got dystentery from germs (bacteria) in their drinking water. This happened because there wasn't any organized way to get rid of sewage, so the sewage got mixed in with the drinking water, and the germs in the sewage also got into the water.
People knew that drinking water was dangerous, but they didn't know why, or how to stop it. A lot of people drank beer or wine or cider instead of water, to try to stay healthy. Dysentery was especially a problem in the summer, because in the winter cold might kill the germs.
If you got dysentery, you lost water from your body faster than you could drink it, and after a few days you might die of dehydration (not having enough water in your body) or of fever caused by the infection. Strong adults usually lived, but babies and children and older people often died of dysentery.