When the Irish crossed the Atlantic, looking for a better life, they travelled on what became known as "coffin ships". It was common for 20-30% of the passengers to die during transit, and sometimes reportedly up to 50%.
As an interesting factoid these sort of mortality figures were not especially uncommon in naval voyages until surprisingly recently. Scurvy is kind of a joke now a days, but it killed millions of people. It was such a big deal that vitamin c is literally named after it - ascorbic acid, or anti-scurvy acid. But that only happened on into the 20th century!
The idea that such a brutal disease could have been prevented by eating fresh fruits and meats sounded more like a folktale than reality. And early experiments to try to demonstrate this were also not that conclusive since vitamin c tends to break down rapidly in the conditions it was stored in (prejuiced - metal containers). For instance during Vasco de Gamma's journey from Europe to India he lost more than half his crew, mostly to scurvy.
And not all that survived the trip across made it much further. Typhoid or "ship fever" was killing a lot of the passengers and when they arrived in North America, they were put into quarantine camps where many died.
