The number of cases and deaths in the coronavirus epidemic in China jumped sharply on Thursday as the authorities there changed how they keep track of the disease, and not, primarily, because of any change in the shape of the outbreak.

There is still a lot of uncertainty about the true numbers — as with any new disease — and that will continue for the foreseeable future. Underscoring that point, hours after China revised its figures, the World Health Organization put out a lower set, saying that for now, it would not change the way it counts.

But experts say there were good reasons for changing the way patients with the disease, newly named Covid-19, were counted, and the new approach should catch many cases that had been missed before. That will allow more patients to get treatment and be quarantined faster.

Here is a look at what we know — and what we don’t — about the new coronavirus numbers and what they say about the scope of the crisis.