If you assume a random person has a probability of carrying the virus at 0.001, here's the probability of someone in a crowd of N people being sick - https://w

If you assume a random person has a probability of carrying the virus at 0.001, here's the probability of someone in a crowd of N people being sick - https://www.wolframalpha.com/input/?i=plot+%281-%280.999%29%5En%29+for+n%3D1+to+2000 https://www.wolframalpha.com/input/?i=plot+%281-%280.999%29%... At around the 700 mark, the probability is > 0.5. So if you are in a crowd of 700+ people, someone's likely sick. Of course, you need to have some reasonable estimate of how many are carrying the virus in a given population to do this calculation.