Well, basically, they died. Dysentery and same kind disease were devastating for medieval armies, especially when besieging a fortress. Their cause wasn't well known, although by simple common sense they tried to avoid camping in the swamps, they couldn't do much about it.

As a consequence, dysentery outbreaks could have huge impact on battles or wars. On of the reason the English were so few against so many French in the battle of Agincourt (Azincourt in original version) was that Henry Vth lead the terrible siege of Harfleur earlier in the year (1415). His army was decimated by diseases due to the swampy nature of the land, which had been flooded. Another exemple sees the same Henry Vth die himself of dysentery. He caught it in the siege of Meaux, in 1422, and died some months later, rather weakened by it.

Dysentery was really a thing to take in consideration, yet its causes (and those of similar diseases, like, say, typhus) were not fully understood. That lead to very high death rates in some occasions, and i mean, VERY high death rates : Henry Vth landed in France with an estimated 30 000 men in 1415. After the siege of Harfleur (were the was very few fiighting and frontal assault), and in the battle of Agincourt, it is estimated he fielded about 12-15 000 men. Quite an heavy toll.