Only around half of the residents of one Washington nursing home overwhelmed by COVID-19 had symptoms when they tested positive for the coronavirus, according to a new report.

King County, Washington, was the first coronavirus hotspot in the United States. Multiple nursing homes in the area had cases, including the Life Care facility where two-thirds of the residents were found to have the virus that has been linked to 37 deaths.

The new coronavirus is spreading through the US, and several states have made emergency declarations. The World Health Organization has declared it a pandemic. Here are the basics: Everything you need to know about the coronavirus

The Centers for Disease Control and Prevention (CDC) gave coronavirus tests to 76 people in a different facility on March 13th, and 23 of those tests were positive. Only 10 of those people had symptoms on the day they were tested, according to data published today in the CDC’s Morbidity and Mortality Weekly Report. The remaining 13 did not appear to have symptoms on the day they were tested, but 10 developed symptoms — including fever and cough — one week later.

After the first person in the care facility was diagnosed with the virus, staff restricted visitors, began regularly screening all residents for symptoms, and isolated those who were sick. The report notes, though, that symptom-based precautions wouldn’t have been enough to stop the spread of disease. Residents who didn’t have symptoms had similar levels of the virus in their respiratory systems as those who did, and they could have passed it to others before it seemed like they were sick.

That’s consistent with recent research done around the world showing that people with mild or no symptoms could be driving outbreaks. Studies show that they have high levels of the virus in their noses and throats and that people appear to be more infectious earlier in the course of their illness before significant symptoms may have developed. COVID-19 is an unusual illness in that respect. When the virus first appeared, experts — including Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases — stressed that asymptomatic cases haven’t been the main driver in other outbreaks of respiratory diseases.

Identifying and controlling COVID-19 outbreaks in nursing homes and long-term care facilities is especially important because their residents — who are older and have other health conditions — are at high risk for having a severe case of the disease. Infectious diseases spread quickly in nursing homes, like they do in other closed environments. Many residents share rooms, and staff and aides go from room to room regularly. Dozens of nursing homes in the US have reported cases. In one New Jersey nursing home, all 94 residents are assumed to have contracted the virus.

The CDC report outlines the steps that these facilities could take to protect against outbreaks if someone gets sick. Because symptoms aren’t a good way to figure out who has the virus, it suggests they do widespread testing. It also recommends that both health care personnel wear masks and other protective equipment.

But those aren’t steps most places in the US will be able to take, which the report also notes. Tests are in short supply and are largely restricted to people with symptoms or who have been hospitalized. Nurses and doctors at hospitals are facing severe shortages of protective equipment and masks. Without those resources, it’s nearly impossible for anyone to follow public health recommendations — including for those people, like nursing home residents, who are the most vulnerable.

The US now has the highest number of cases of COVID-19 in the world, and experts predict the outbreak will only get worse.