Officials said Friday the latest victim was a Minnesota boy who died Sept. 7, The Washington Post reported.
The newspaper said the boy was the 19th patient at the Maryland hospital to be infected with Klebsiella pneumoniae. He is the first new case since January.
The bacteria's appearance was traced to a New York woman who was treated at the facility in August 2011.
"It's heartbreaking," clinical center Director John Gallin said. "What happened this summer was a very unfortunate case. All of these cases are hugely sad cases."
The boy had gone to the Bethesda hospital in April for treatment of complications from a bone marrow transplant.
The hospital has gone through extensive efforts to try to wipe out the superbug only to have the new case crop up, the Post said.
"This kid probably got this infection because a patient who was a carrier [of the superbug] was on the same unit," said Gallin. "There was undoubtedly some intra-hospital transmission despite our best efforts."