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"Certainly, if you're in very close contact with somebody who's shedding a lot of virus, and you happen to get a droplet on your hand and then wipe your nose, that could take far less than 15 minutes" to infect you, says Angela Rasmussen, a virologist at Columbia University Mailman School of Public Health.
How you interacted also matters a great deal, Barocas says. For example, was the infected person coughing? Was the person wearing a mask, which can help contain a lot of the infectious particles someone might be breathing out? Were you indoors or outdoors, where airflow would quickly disperse any infectious particles the person might have exhaled? How infectious was the person at the time of interaction? (Studies have shown that people with the coronavirus are most infectious just before and in the first few days after they start to show symptoms.) If an infected person were to cough on you while walking past, that would constitute a high-risk interaction – even if it was brief, he says.