On a sunny September morning in 2011, at a conference on the Mediterranean island of Malta, virologist Ron Fouchier made an announcement that shook the scientific world.

His lab, he said, had taken the H5N1 avian influenza virus — which kills around 60% of people with known cases, but which cannot spread easily from person to person — and altered it to transmit among mammals.

He had created, he told a reporter later that year, “probably one of the most dangerous viruses you can make.”

Fouchier and others have said such research can help scientists prepare for future pandemics. But several thousand miles away in Massachusetts, Lynn Klotz reacted with concern.

A physical biochemist, Klotz was on the Harvard University faculty in the 1970s, during contentious disputes about recombinant DNA research. Since 2005, he has been a senior science fellow at the nonprofit Center for Arms Control and Non-Proliferation, where he has written about biological weapons.