As the number of coronavirus cases jumps dramatically in China, a top infectious-disease scientist warns that things could get far worse: Two-thirds of the world’s population could catch it.

So says Ira Longini, an adviser to the World Health Organization who tracked studies of the virus’s transmissibility in China. His estimate implies that there could eventually be billions of more infections than the current official tally of about 60,000.

If the virus spreads to anywhere near that extent, it will show the limitations of China’s strict containment measures, including quarantining areas inhabited by tens of millions of people. WHO Director-General Tedros Adhanom Ghebreyesus has credited those steps with giving the rest of China and the world a “window” in which to prepare.