In My Brother Sam Is Dead, what does Tim's father die from on the prison ship?

My Brother Sam is Dead is a work of historical fiction by James Lincoln Collier and Christopher Collier that targets young adult readers. The book traces the life of Tim Meeker, a boy living in Connecticut just before the dawn of the American Revolution, who must deal with the wildly different political views of his father, Eliphalet "Life" Meeker, (who is loyal to Great Britain) and his brother Sam (a member of the Continental Army). Tim watches as the conflict between these two--and between the countries they believe in--continues to escalate to the point of senseless violence. 

Eventually, Tim's father, is captured by Rebels who believe that he has been selling beef to New York loyalists. He later dies on a prison ship after an outbreak of cholera.