Dysentery is a nasty sounding word for a nasty condition: an intestinal infection that results in diarrhea.
There are few things as unpleasant as dysentery, an infectious condition, kind of like the flu, that includes severe diarrhea. Dysentery is common in countries and places with poor health conditions and sewage problems. If the drinking water is filthy, that's a great way to spread dysentery. This word sounds kind of dirty, and that's a good hint to its meaning: the horrible illness of dysentery spreads in dirty, unsafe living conditions.