A new CDC study suggests roughly 50% of people living with someone who has COVID-19 get it — usually in less than 5 days

• A new CDC study suggests it's very easy to get the coronavirus from someone who's living in your household.
• The report showed that roughly half (53%) of people surveyed who were living with a COVID-19 positive person wound up sick within a week, according to their daily self-administered tests.
• Illnesses were transmitted quickly, with 75% of infections being passed along in five days.
• The study authors said that people "who suspect that they might have COVID-19 should isolate, stay at home, and use a separate bedroom and bathroom if feasible."

A US Centers for Disease Control and Prevention (CDC) report released Friday suggests that getting the coronavirus from someone you live with can be quick and easy, no matter their age.

The study, which is ongoing in over 100 households in Nashville, Tennessee and Marshfield, Wisconsin since April, found that roughly half (53%) of study participants living with a sick person who tested positive for COVID-19, the disease caused by the coronavirus, wound up sick themselves within a week. 75% of those secondary cases tested positive for the virus within five days or less, according to their daily, self-administered tests.

"Persons who suspect that they might have COVID-19 should isolate, stay at home, and use a separate bedroom and bathroom if feasible," the study authors wrote in their report, stressing that isolation should start as soon as the person suspects that they might be sick, even before any testing is done.

Being in the same room with a sick person is dangerous

In the study, most sick patients said they had spent many hours (four or more) together in the same room with the people they live with on the day before they started feeling unwell. That pre-symptomatic period is exactly when health experts suspect that people with the virus are at their most infectious.

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"It's because the disease can spread at that moment that the disease is so contagious," the World Health Organization's Executive Director of Health Emergencies, Mike Ryan, said earlier this year. "That's why it's spread around the world in such an uncontained way."

Another factor working against people who share a home with sick patients: airflow. The coronavirus spreads well between people who are indoors, and gathered close together, in poorly-ventilated spaces, so it makes sense that people would be getting infected from those they live, breathe, sleep, and eat with every day.

"We know that the biggest risk is these closed, indoor environments," University of Maryland virologist Don Milton previously told Insider.

(However, as the study authors noted, it is always possible that some of the participants might've gotten infected in some other way.)

In the study, 40% of sick patients were sleeping in the same room as another person in their household, before they knew they were sick. The ages of the study participants ranged from younger than 12 to older than 50 years old. It was possible for household members to opt out of the study, which "did happen, but infrequently," study author Melissa Rolfes told Insider, in an email.

Put on masks if you have to share space with sick people at home

The study authors suggest that both the sick person and all the people in their household should start wearing masks as soon as they think they might have the virus, "particularly in shared spaces where appropriate distancing is not possible." You may also want to open up some windows (if it's not too cold) or get some fans moving, to improve air circulation and blow virus particles away.

If you live with someone who has the virus, you should also stay away from others who don't live in your house for two weeks, in quarantine. This is because it's possible that you could have contracted the virus, and could be capable of spreading it to others, without ever knowing you're contagious. Indeed, in the study, only 40% of sick patients' housemates who subsequently fell ill had any symptoms when their infection was initially detected by a test.

"Usually if you were to develop symptoms, it's going to be between, let's say, two and 12 days," Dr. Rishi Desai, chief medical officer at Osmosis, and a former Epidemic Intelligence Service officer at the CDC, recently told Insider.