That's why I try not to think about it and just focus on keeping my hands and face clean, and when you wear glasses it is hard to keep from touching your nose, so I am conscious of it. We could all become Mr. Monk!
It was easier with radioactive contamination. We could detect it on the person when they alarmed a monitor, then we could backtrack their pathway and we'd find it on the door handle they touched with their 'crapped up' hand, and so forth. Too bad there isn't a detector for norovirus.