Possibly the most worrying thing about COVID-19 is that a person can have it and be completely symptom-free. On the surface, that seems great. How could not having symptoms be bad? If it were just that it didn’t make you ill, that would be great. However, people who have no symptoms can still pass on the virus to others. Researchers believe that around 45 percent of people who become visibly ill were infected by someone who seemed completely healthy.

Scientists from the Scripps Research Translational Institute looked at 16 different groups of COVID-19 patients worldwide to learn the number of people who spread the virus, unaware of the fact that they had it. They think that, at minimum, 30 percent were caught from asymptomatic people, and it was probably closer to 40 or 45 percent.

“The range we found is extraordinarily high,” says Dr. Eric Topol, director and founder of the Institute. “That means the range of what can happen with SARS-CoV-2 is from no symptoms to [death]. That’s not at all similar to any virus or pathogen we’ve experienced that has killing potential in the past. What we have here is an extraordinary spectrum, including this quiet, stealth mode of infecting somebody.”

In one group of 2,300 people who were sampled, 41 percent of people who had the virus never developed symptoms — but they were contagious. A different study found that asymptomatic people could still be left with lung damage despite never realizing they were ill.

“The priorities during a pandemic are absolutely to look after the sick,” said Dr. Topol. “But we also shouldn’t miss how important this area of asymptomatic spread is to understand. For every one person who is sick, there are a whole lot of people who have the virus and don’t know it.”

Scientists have been saying it’s essential to wear masks to prevent ourselves from spreading the virus. This new study underscores that. You may appear completely healthy and still be a danger to others. And when other people wear masks, they are protecting you.

“When it comes to controlling Covid-19, this really shows that we cannot rely on self-isolation of symptomatic cases only,” said Dr. Rein Houben at the London School of Hygiene and Tropical medicine. “Going forwards, we need trace and test approaches to account for individuals who are not reporting any symptoms.”

Not feeling ill may make you feel like you don’t need a mask or socially distance yourself. However, with so many asymptomatic people, there is no way of knowing that you are safe. The virus is so different than other illnesses. Only five percent of people who get the flu are asymptomatic. The coronavirus is both more deadly and easier to miss than flu. So, until we gat the all clear, it’s best to steer clear of people you don’t live with and limit your contact with others.