The coronavirus pandemic has exposed a clash among medical experts over disease transmission that stretches back nearly a century — to the very origins of germ theory.

The Geneva-based World Health Organization acknowledged this week that the novel coronavirus can spread through tiny droplets floating in the air, a nod to more than 200 experts in aerosol science who publicly complained that the U.N. agency had failed to warn the public about this risk.

Yet the WHO still insists on more definitive proof that the novel coronavirus, which causes the respiratory disease COVID-19, can be transmitted through the air, a trait that would put it on par with measles and tuberculosis and require even more stringent measures to contain its spread.