I think that's like the flu or common cold too, I think sometimes people get those and never know it too because why would you know it if you don't have any symptoms. Or maybe for one day you have the sniffles, you might just assume that is allergies or more likely not even think about it for a second. I think it depends on viral load, you might breath in just a few molecules of virus and your immune system easily deals with it. Or you might breath in a shit load and that overloads the immune system.