Given the mounting economic risks posed by the spread of the novel coronavirus, Congress should act swiftly but thoughtfully to pass fiscal stimulus. This would be in addition to continuing to provide ample funding for medical research, testing, prevention and treatment. The stimulus’s total cost would be about $350 billion, but could be larger or smaller depending on how the economic situation unfolds. Congress should design it to be accelerated, big, comprehensive and dynamic.

• Accelerated. In normal times it takes a year...