From the New York Times
….in the case of the new coronavirus, people who have no symptoms seem to have viral loads — that is, the amount of virus in their bodies — just as high as those who are seriously ill, according to some studies.
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.
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?
And coronavirus patients are most infectious two to three days before symptoms begin, less so after the illness really hits.
Some people are generous transmitters of the coronavirus; others are stingy. So-called super-spreaders seem to be particularly gifted in transmitting it, although it’s unclear whether that’s because of their biology or their behavior.
On the receiving end, the shape of a person’s nostrils and the amount of nose hair and mucus present — as well as the distribution of certain cellular receptors in the airway that the virus needs to latch on to — can all influence how much virus it takes to become infected.