New testing results out of Iceland indicate that asymptomatic carriers may well be everywhere: In tests of 9,000 people, a whopping 50% of the positive cases were in people reporting no symptoms. (There’s a bit of a sample bias here as these participants were self-selected volunteers, not the general population.)

Asymptomatic people—carriers who will never develop symptoms—have been completely off the radar of U.S. public health officials. Think about it: If Bob infects you with COVID-19, officials will not be aware of Bob’s positive status unless he gets tested, so experts have no idea how many asymptomatic Americans are huddling down with their families, nor how often they might transmit COVID-19. Asymptomatic carriers are only identified with mass testing.

Presymptomatic people are indeed spreading illness. The CDC looked at new cases in Singapore over a seven-week period, and determined that a significant percentage (6.4%) of new local cases came from presymptomatic transmission in seven clusters. This is bolstered by a reports of U.S. clusters, like a Massachusetts 82-person cluster started by presymptomatic patients.

Folks, this is why we’re social distancing. Perfectly healthy-seeming people can definitely transmit COVID-19. (In CDC-speak, “presymptomatic transmission underscores the importance of social distancing, including the avoidance of congregate settings.”)

These are all early reports based on small samples, and so expect the numbers to shift significantly. But the take-home message is clear: People who don’t seem sick can spread COVID-19.