Developments from the scientific community

The urgency of the crisis has led to a new level of global scientific collaboration: Never have so many experts in so many countries focused simultaneously on one topic. What we know about the virus, and how to treat it, is expanding every day.

We are also learning a lot more about what we don’t know — like exactly who has the coronavirus. The Centers for Disease Control and Prevention now believes that as many as 25 percent of infected people may show no symptoms, significantly more than earlier estimates.

“This helps explain how rapidly this virus continues to spread across the country,” the agency’s director, Dr. Robert Redfield, told NPR on Tuesday.

Moreover, research out of China has found a high rate of false negatives: Around 30 percent of people who are tested and told they are negative may actually have the virus. It may be necessary to test twice to be sure.

In an article for The Times, Dr. Harlan Krumholz, a professor of medicine at Yale, suggests that “if you have had likely exposures, and symptoms suggest Covid-19 infection, you probably have it — even if your test is negative.”

Some good news in drug trials: A small study of 62 patients in China with mild cases showed promising results for the malaria drug hydroxychloroquine: patients who received it seemed to get over their symptoms faster.

And doctors around the world noticed that some Covid-19 patients seemed to experience “cytokine storms,” which is when their immune systems kept raging after the virus was defeated. That led them to try the drug tocilizumab, with early signs of success. The F.D.A. has approved testing in the U.S.