Most emigrants from the Irish Famine (the great hunger) died on boats why was this? thanks?

I had an Great grandmother and her sister come to America a few years after that. Her sister died of something on the ship, maybe dysentery or something. That was probably common. Also things like smallpox, typhoid, etc. could sweep through a ship. Many people were weakened from hunger and had things like TB. I don't know if 'most' died, but I'm sure there were a lot who did not make the trip very well. Those who did survive were the stronger ones. (I don't like 'survival of the fittest', but it might apply)