health officials have changed their advice on face masks and now recommend people wear cloth masks in public areas where social distancing can be difficult, such as grocery stores?

The World Health Organization and the CDC have repeatedly said that most people do not need to wear masks unless they are sick and coughing.

surgeon general even urged the public to stop buying medical masks, warning that it would not help against the spread of the coronavirus.

But a growing body of data now suggests that a significant number of infected people who don't have symptoms can still transmit the virus to others.

It describes how the testing of passengers and crew on board the Diamond Princess found that nearly half — 46.5 percent — of the more than 700 people found to be infected with the new coronavirus had no symptoms at the time of testing.

Harvey Fineberg, former president of the National Academy of Medicine and head of a new federal committee on infectious diseases, told CNN on April 2 that he will start wearing a mask in public, especially at grocery stores, for this very reason.

It is these "silent carriers" — people infected with the virus but without fever, cough, or muscle aches — that proponents of universal mask wearing point to as proof that more could be done beyond social distancing to slow the virus's spread.

While research on the effectiveness of universal mask wearing for reducing respiratory droplet transmission is still thin, there is evidence to support it.

Research on SARS, another coronavirus, found that N95 masks were highly effective at blocking transmission of that virus.

The surgical masks that doctors and nurses typically wear are designed for one-time use, while cloth masks used by the general public would likely be washed, which raises another concern.

A study from Nepal on cloth masks designed to protect wearers from larger particles, such as pollution or pollen, found that washing and drying practices deteriorated the mask's efficiency because they damaged the cloth material.

It is clear that urgent research is needed on the best material suitable for universal masks, their storage and care, or the creation of proper reusable masks for the public.

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