We need to ban some gain-of-function research on deadly viruses. We also need a better definition of what it is we're banning.

What exactly is gain-of-function research, and is it cause for alarm? Researchers have been modifying viruses for decades—and not always in benign ways.

Gain-of-function research is the serial passaging of microorganisms to increase transmissibility, virulence, immunogenicity, and host tropism.