In the spring of 2020, COVID-19 ripped through his household. By May, Castillo wound up in the hospital and was put on a ventilator.
Though free of COVID, the virus essentially destroyed his lungs. The organs simply couldn't recover.
A rare and risky double lung transplant turned out to be his only hope for survival.
"Transplant should always be the option of last resort," said Dr. Ankit Bharat, a thoracic surgeon at Northwestern Memorial Hospital in Chicago.
Bharat was the first surgeon to perform a double lung transplant on a coronavirus patient in the U.S. Before, only a few other COVID-19 survivors, in China and Europe, have received lung transplants.
Just six days after Castillo was listed for transplant, Northwestern found a donor.
After Bharat performed the harrowing surgery, Castillo survived but is still not out of the woods. He's in the middle of a long, brutal recovery.
"I want to say thank you. You are the best, you are the best," Castillo told Bharat.
Castillo is one of 12 COVID survivors who received a double lung transplant from Bharat. He's collaborated with hospitals throughout the country, in total contributing to 50 of these life-saving procedures.