Sounds like Oregon Trail was pretty spot-on when it came to causes of death. When Pa succumbed to the big D, the digital avatar was living the pain of tens of thousands of American pioneers.
There are two forms of dysentery. One is caused by a bacterium, the other, an amoeba. The former is the most common in Western Europe and the United States; and is typically spread through contaminated food and water.
Outbreaks of dysentery were more prevalent during war, where the disease spread rampantly because of the unhygienic conditions of the camps. During the Mexican War (1846-48), a staggering 88% of deaths were due to infectious disease, most of those overwhelmingly dysentery. For every man killed in battle, seven died of disease. The American Civil War was no better. You were more likely to die off the battlefield than on it, and dysentery was the primary cause. 
That said, civilians also died of dysentery with some frequency in the 19th century, especially those who were itinerant. Pioneers travelling the Oregon Trail wouldn’t have faired much better than soldiers fighting in war.
Read more about this microbial menace of yesteryear at thechirurgeonsapprentice.
Now about that whole caulk-the-wagon-ford-the-river thing …