Of the diseases mentioned specifically by name in Anglo-Saxon sources, dysentery stands out. Dysentery is mentioned in Bede’s Life of Cuthbert (Ch. 8), where he says that Bishop Eata died of “a disease that the doctors call dysentery”. He is telling us this to distinguish Eata’s later death from Boisil’s death from the plague. This is particularly important because Eata dies in c. 685 in the midst of another wave of plague, or at least what we think was plague. It is a good reminder that people died of other infections also during waves of plague.
One of the continuers of Bede’s chronological summery gives us a second more complete reference to dysentery.
“759: Oswulf was treacherously killed by his thegns and in the same year Æthelwold was elected by the people and began his reign. In his second year a great pestilence occurred and lasted nearly two years. The people were wasted by various kinds of malignant diseases but especially dysentery.” (McClure and Collins, Ecclesiastical History of the English People, p. 297)
Tough times in Northumbria. Epidemic dysentery is always an issue of water quality. The northern version of the Anglo-Saxon Chronicle mentions that 761 had a very hard winter. We might imagine in these circumstances that people stayed indoors more often and ice may have prevented the removal of human waste and made it harder to get fresh water. Epidemic dysentery is almost always an issue of water or food contamination; food being often contaminated by poor water. Large monasteries may have frequently had issues with human waste removal and water contamination.
Dysentery, from their perspective, would refer to any type of bloody diarrhea. It is sometimes called the bloody flux. There can be multiple causes, bacterial and amoebic. When a chronicle mentions dysentery I think it must be epidemic dysentery rather than sporadic, which would not be worth mention. Epidemic dysentery is almost always bacterial. Today, epidemic dysentery is usually caused by a Shigella species, sometimes called Shigellosis today. However there are multiple species including E. coli that can cause a bloody diarrhea. Ameobic dysentery is caused by Entamoeba histolytica, and would usually be found at a constant low level where it occurs. Once any of these get established in a community it would be difficult to get rid of them, especially considering levels of personal and community hygiene. For example, you have to be diligent about hand washing and cleaning infected infants diapers to get Shigella under control. Dysentery is most dangerous to those most vulnerable to dehydration; the old, the very young and those suffering from malnutrition. Rehydration is the best treatment and often the only treatment necessary even today.