4. Why is this a problem?

Some researchers have suggested that unrecognized carriers may be a reason why this coronavirus has spread extremely quickly relative to others, such as the one responsible for severe acute respiratory syndrome, or SARS, almost 20 years ago. Current containment measures are predicated on early detection and isolation, but patients with mild, non-specific, or no symptoms are more difficult to identify. Such cases can also increase the risk to health-care workers if a patient undergoes certain high-risk medical or surgical procedures, such as a colonoscopy. Fourteen hospital staff were infected after a patient in China tested positive days after surgery, and they had failed to take necessary infection-control precautions.

5. What can we do about it?

Firstly, more community testing, especially of people known to have been in contact with a confirmed case. Where tests aren’t widely available, X-rays may help. Italian researchers found chest images can be used to identify probable cases among people displaying mild or no symptoms. Secondly, some health officials recommend wearing face masks or cloth barriers covering the nose and mouth to minimize both the projection of virus-carrying respiratory secretions from an infected person and the risk of a non-infected person inhaling virus particles. Thirdly, practice social distancing and avoid congregating in large groups.