Enrique Mirabal died as a result of his harsh treatment in prison and his failing health owing to the heart attack he had there. Though he doesn't die until 1953, this family still says that it's what he went through in prison that killed him.
When the family is in prison, they find out that their father was taken to the hospital there. The diagnosis is listed as confidential, and so they aren't sure what's wrong, but their belief is that his ulcers are the problem. Unfortunately, he had a heart attack, and it wasn't just his ulcers flaring up.
Even though this occurred in 1949, his daughters still say it's the reason he died. Julia Alvarez writes:
All her sisters looked at her, shocked. "Papà was a hero!" Dede fumed. "He died because of what he went through in prison. You should know. He was trying to keep you out of trouble!"
Since he had a heart attack on a Wednesday and wasn't allowed to see the doctor for five days afterwards in the prison, it seems likely that it would have had lasting effects on his health. When his family is allowed to see him again after the hospital, they see that he looks ill, gaunt, and his clothes hang on him because they no longer fit. He's never the same after what happens in the prison.