The University of California San Francisco has voluntarily shut down its living donor program after a kidney donor died unexpectedly.

In what has been called a nightmare scenario, the donor provided a kidney to a recipient at UCSF Medical Center back in October, but died shortly after while the recipient recovered fine, prompting an investigation by hospital and regulatory officials, according to an SFGate.com report.

Dr. Steven Katznelson, who is the medical director of California Pacific Medical Center’s kidney transplantation program in San Francisco, said that many doctors worry about this very scenario when performing the transplants. All surgeries carry a small risk of complications or even death, and while transplants are relatively safe, they can have these results on occasion.

A person who donates a kidney faces only a 0.03 percent change of dying. While most kidney transplants come from a recently deceased person, living donors give a person a better chance of surviving.

A total of two deaths were recorded in 2014, and two have so far been reported in 2015, including the UCSF incident.

A total of 350 kidney transplants are performed each year at UCSF, and a total of 10,000 have been performed since 1964. About 150 of the 350 kidney transplants annually typically involve living donors.

“Most kidneys for transplant are from people who have died and whose families give permission for organ donation. But there aren’t enough of these organs for everyone who needs one,” UCSF says on its website. “Nationally, more than 70,000 patients are on the kidney transplant waiting list, and more are added each year. When a kidney is transplanted from a living donor, the donor’s remaining kidney enlarges to take over the work of two. As with any major operation, there is a chance of complication. But kidney donors have the same life expectancy, general health and kidney function as others.”