With New York being the epicenter for the virus in the U.S., researchers from the state’s largest health care system moved quickly to document findings that would help others identify and treat COVID-19 patients. One thing stood out in the study published in the Journal of the American Medical Association.

“The most surprising finding to me was that two-thirds of the patients who were seriously ill with an active infection did not have a fever,” said senior researcher Karina Davidson, PhD.

Fever is the first symptom doctors look for, but Davidson said the sickest patients didn’t present with a fever.

“This is a puzzling infection,” she said. “Different people have different symptoms, some of them mild, some of those severe.”