When experts recommend wearing masks, staying at least six feet away from others, washing your hands frequently and avoiding crowded spaces, what they’re really saying is: Try to minimize the amount of virus you encounter. Apoorva Mandavilli writes in the New York Times that a few viral particles cannot make you sick — the immune system would vanquish the intruders before they could. But how much virus is needed for an infection to take root? What is the minimum effective dose? A precise answer is impossible, because it’s difficult to capture the moment of infection.