A virus specific to pigs may have been the reason a man who received the first successful transplant of pig heart died two months after the transplant, the operating physician said, according to MIT Technology Review.

“We are beginning to learn why he passed on,” Bartley Griffith of the University of Maryland School of Medicine said during an American Society of Transplantation webinar in April, MIT Technology Review reported.

The heart was affected by porcine cytomegalovirus, which “maybe was the actor, or could be the actor, that set this whole thing off," Griffith said.

Pigs raised to provide organs to humans are supposed to be virus-free, MIT Technology Review reported.

David Bennett received the genetically modified porcine heart on Jan. 7 after the Food and Drug Administration gave emergency authorization on New Year’s Eve.

Before the transplant, Bennett had been hospitalized for six weeks with a life-threatening arrhythmia and had been connected to a heart-lung bypass machine.

Bennett’s transplant was a milestone in the field of xenotransplantation, the sourcing of animal organs to address the human organ supply crisis.

He died two months after the transplant. At the time the cause of Bennett’s death wasn’t immediately disclosed, and doctors at the University of Maryland Medical Center didn’t say whether it was connected to any complications from the transplant.

Pig and cow tissues have been used successfully for valve replacements, according to Harvard University Medical School. Those valves typically last about 15 years and don’t require the use of anti-clotting drugs as opposed to a mechanical valve, which can last the rest of a person’s life.

Xenotransplants, the term used for transplants from a nonhuman species to a human, have been researched as the demand for organ replacement increases. According to the FDA, 10 patients die per day while awaiting a donated organ. 

Elisha Fieldstadt is a breaking news reporter for NBC News.