Why was disease the leading cause of death of the enslaved africans on the middle passage

Since Slaves we're crammed into ships, they often died of disease, as diseases spread much more effectively when people are constantly in contact with each other, as they often were because so many slaves were fit onto each ship. Think about it: You're more likely to catch a cold if you get coughed on no matter what you do, because you are always close to the sick person.