“Theoretically, you can manipulate almost any virus to change it in any way you want,” said Fauci, whose agency is part of the National Institutes of Health.

“The only trouble is, it wouldn’t be easy for somebody to do that in their backyard laboratory. They would probably kill themselves doing it. It would take a state-type [actor].”

The Senate hearing comes as policymakers consider ways to counter the Islamic State in Iraq and Syria, a terrorist army that is gaining ground in the region.

The group, known as ISIS, was not mentioned at Tuesday’s hearing.