Last night on 'The O'Reilly Factor,' Bill O'Reilly spoke with Dr. Tomas Villanueva, who has been treating former U.S. Marine Jon Hammar in Miami.

Hammar, 27, spent four months in a rundown Mexican prison after being arrested at the border when he tried to enter the country with an antique shotgun. For some of his incarceration he was chained to his bed.

When he was released from the prison two weeks ago, Hammar's father began driving him back to south Florida from Texas. But Hammar was too ill to go very far on the trip, so his father stopped

at a hospital in Lafayette, La., where Hammar was admitted with a fever, severe diarrhea, vomiting, and headaches.

Related Posts:

Villanueva explained that he believes the diarrhea began about seven to 10 days before Hammar was released from the prison. He said the extreme dehydration brought on by the diarrhea led to other issues.

"He did not have access to enough food and water (in the prison)," said Villanueva, who added that Hammar was found to have multiple mosquito bites all over his body, had lost 15 pounds and overall "was in a bad state."

The good news is that, according to Villanueva, the prognosis for Hammar is positive and he should make a full recovery. But Villanueva explained that one in five deaths around the world occur because of diarrhea, so "it's very fortunate" that Hammar got out of prison when he did.

"If he continued with these symptoms, conceivably speaking we could have lost Johnny," said Villanueva.

Watch the interview:

Follow Fox News Insider, the official blog of Fox News Channel on Twitter and Google+!