These mucus and saliva droplets are ejected from the mouth or the nose as we cough, sneeze, laugh, sing, breathe and talk. If they don’t hit something along the way, they typically land on the floor or the ground. When the virus becomes suspended in droplets smaller than five micrometers — known as aerosols — it can stay suspended for about a half-hour, research suggests.

To gain access to your cells, the viral droplets must enter through the eyes, the nose or the mouth. Some experts believe that sneezing and coughing are most likely the primary forms of transmission. Professor Kwok said talking face-to-face or sharing a meal with someone could pose a risk.

Julian Tang, a virologist and a professor at the University of Leicester in England who is researching the coronavirus with Professor Kwok, agreed.

“If you can smell what someone had for lunch — garlic, curry, etc. — you are inhaling what they are breathing out, including any virus in their breath,” he said.

The virus does not linger in the air at high enough levels to be a risk to most people. But the techniques health care workers use to care for sick people can generate high levels of aerosols. This is part of why it’s so important that they have proper protective equipment.

How close is too close?

The Centers for Disease Control and Prevention recommends keeping a distance of six feet from other people to minimize the possibility of infection. (A useful way to think about six feet is that it’s roughly twice the length of the average person’s extended arm.)

Three feet is the distance the W.H.O. emphasizes as particularly risky when standing near a person who is coughing or sneezing.