The source of a lethal case of rabies was traced back to an infected kidney transplant 18 months prior.

The Maryland man died of the encephalitis-type infection, similar to the donor's cause of death, Health reported.

The donor, a 20-year-old serving in the Air Force, died from "unexplained brain swelling." Medical researchers originally suspected food poisoning, but water testing proved that scenario to be unlikely.

After performing an autopsy the doctor's said the cause of death was gastrointestinal inflammation, they never imagined it was rabies.

Organ donors are tested for diseases like: "HIV, herpes, hepatitis and cytomegalovirus."" Rabies screening is rare because there are very few reported cases of the disease and the tests are time consuming.

Since symptoms didn't appear for so long in the man who received the transplant, doctors weren't on the lookout for rabies. 

"I think it's important to approach the donor who might have encephalitis a little more broadly and definitely probably consider rabies as a potential cause of it," Dr. Sridhar Basavaraju, a member of the investigating team, said.

Friends and family of the originally infected man said he regularly trapped raccoons to use as bait during dog training; he had been bitten by the raccoons at least twice.

Raccoons experience the highest number of rabies cases among animals, but there has only been one other case of the infection being passed on to a human in the U.S.

Three other people who recieved the infected organs were notified and given shots, they are reportedly doing well, Health reported.

"The rabies virus is transmitted through saliva or brain/nervous system tissue. You can only get rabies by coming in contact with these specific bodily excretions and tissues," The Centers for Disease Control and Prevention (CDC) reported.

The first symptoms of the disease include flu-like symptoms, and later "cerebral dysfunction, anxiety, confusion, agitation." During the final stages of rabies people may experience "delirium, abnormal behavior, hallucinations, and insomnia."

About one or two people in the U.S. are infected with rabies every year, only 10 have ever survived after exhibiting symptoms.