You might walk into stores over the next few days and sicken dozens without knowing it. Some might die. Others will think they are dying before they recover.

That’s the worry I have after reading a paper by Roman Wölfel and colleagues, published this week in Nature. It shows that people are most infectious in the first week after catching Covid-19. During that time they often show no or few symptoms.

In other words, Covid-19 moves like a silent assassin, with unwitting accomplices. Maybe you’ll be one of them. The best way to ensure that you’re not: wear a mask, and keep your distance from others. Don’t wear an N95 respirator, the type in desperately short supply in hospitals, which is designed to keep doctors safe even when doing potentially dangerous medical procedures. But almost any kind of simple cloth covering over your mouth, such as a home-made mask, or even a bandanna, can stop the assassin in its tracks.

The Wölfel paper explains we must focus our efforts on stopping the spread of droplets. This is because the virus is primarily transmitted through tiny droplets of saliva ejected when we speak. You can’t see them, but they are there. We also know that these droplets can go significantly further than the 6ft which is widely cited as a safe distance.

Research supported by Nobel prize-winning virologist Harold Varmus tells us that placing a layer of cloth in front of a person’s face stops 99% of the droplets.

So, the science is clear. We do not know when we are sick. If we are sick, then when we speak we are projecting virus-laden droplets into the air. Wearing a simple cloth mask stops those droplets in their tracks.