The federal Centers for Disease Control and Prevention says it's digging deeper into the case of a rabies-infected organ donor and the death of an organ recipient in Maryland earlier this year.
Rabies expert Richard Franka says that investigators in Atlanta are trying to understand why three of the four people who got organs from the Florida donor in 2011 didn't develop the deadly disease. A fourth recipient from Maryland died of rabies in February.
In a similar case in 2004, all four transplant recipients died of rabies within weeks.
Franka says the answer could lie in the strain of rabies involved, the amount of virus in the transplanted organs, the medical history of the recipients or their genetic makeup.
He says the three survivors, in Florida, Georgia and Illinois, are completing post-exposure treatment and are very likely out of danger.