A patient who recently died of rabies in Maryland contracted the infection from an organ transplant, the U.S. Centers for Disease Control and Prevention confirmed Friday.

Humans rabies cases are unusual in the U.S.; rabies spread by organ donation is even rarer still.

Three other patients also received an organ from the same donor. Those patients are currently receiving anti-rabies shots and being monitored by their doctors.

The Maryland Department of Health and Mental Hygiene began an investigation after the organ recipient died after showing signs of rabies. An autopsy confirmed the rabies infection, even though the recipient had had no exposure to infected animals.

They began investigating the possibility the recipient contracted rabies from the organ donor.

CDC laboratory tests recently confirmed the rabies was transmitted through the organ transplant performed more than a year before the recipient’s death.

The CDC says it’s unusual for a patient to take a year to develop signs of rabies, noting the typical incubation period is one to three months. But they say there have been a few other reports of long incubation periods.

It appears that the organ donor became ill and died of rabies in 2011. The Washington Post reports the donor was a man in his 20s who died of encephalitis. Rabies wasn't suspected as the cause of the fatal brain inflammation, so testing wasn't performed. The donor’s kidneys, heart, and liver, were recovered and sent to patients needing transplants in Florida, Georgia, Illinois, and Maryland.

The CDC says it’s now working with the healthcare facilities where those surgeries took place to find those who were in close contact with the donor and the organs, since they might need rabies treatment as well.

Rabies is typically transmitted to humans through the bite of an infected animal. The virus then infects the central nervous system, where it can take several months to incubate. Once an active illness begins, the infection causes brain disease and death within days.

Human cases of rabies are relatively rare in the U.S., with only one to three cases diagnosed each year, the CDC says. This is Maryland's first case of human rabies since 1976, state public health officials say.

The CDC says while organ procurement organizations regular conduct testing for infectious diseases, such as HIV and hepatitis in donors, testing for rabies is not routine.

“If rabies is not clinically suspected, laboratory testing for rabies is not routinely performed, as it is difficult for doctors to confirm results in the short window of time they have to keep the organs viable for the recipient,” the CDC said in a media release.

The recipient and the donor both had a raccoon type of rabies, though the type can infect other wild and domestic animals. The CDC says it’s aware of only one other person in the U.S. who has died from a raccoon-type rabies virus.