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 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.

Quarantines may slow the spread, but the virus had the opportunity to roam in China and beyond before they went into effect, Longini said. The country boosted its count of those infected by almost 15,000 on Thursday after widening the diagnosis methods.

Longini’s modeling is based on data showing that each infected person normally transmits the disease to two to three other people. A lack of rapid tests and the relative mildness of the infection in some people also makes it difficult to track its spread, he said....