Three people have died from rabies after receiving transplanted organs from a donor with the disease, US health officials have said.
The disease is normally spread through bites from rabid mammals
It is believed to be the first time the disease has been spread in this way.
The Centers for Disease Control and Prevention said the organ donor had been given a routine screening, which does not include testing for rabies.
The donor's lungs, kidneys and liver were transplanted into four patients, one of whom died in surgery.
The deadly disease is more commonly transmitted by the bite or scratch of a rabid mammal.
The donor, a man from Arkansas, died of a brain haemorrhage and his organs were transplanted on 4 May.
Dr Mitchell Cohen, director of the CDC co-ordinating center for infectious diseases, said the man had shown no sign of rabies before his death.
The organs were given a routine screening, which included testing for Hepatitis B and C, HIV and syphilis. Rabies is not part of that process, said the CDC.
The organs were then donated to four patients in Texas, Oklahoma, Alabama and Arkansas.
"This is a very sad and tragic situation, especially for the families of those who lost a loved one," he told a press conference.
"It's also very scary for all the health care workers and family members who may have come into contact with the disease and who are concerned about their health."
Mr Cohen said the CDC was working with health officials in the four states to determine if other people had been infected.