Why Were the Farmers Immigrating from Ireland in the Late

Why were the farmers immigrating from Ireland in the late 1800s particularly prone to disease?

A) They had not been previously exposed to other European and North American pathogens.
B) Traveling on a ship in general increases your risk of disease, as we see in present day outbreaks on cruise ships.
C) Farmers, in general, are not exposed to as many diseases as more urban inhabitants and so do not build up the same immunity.
D) They were poor, undernourished, and weakened in the first place, and the crowded, dirty conditions during their travels made matters worse.
E) They weren't, really. They were just in the wrong place at the wrong time.