If you test for any viruses you will always find them. There are 100 viruses circulating at all times, and we are always carrying some. It's essentially meaningless because a few viruses can be contained and destoyed by our immune systems. If we get weak, tired, those viruses have a chance to multiply and cause illness. Only at that point are we contagious. It's almost as if we've known this all along for a hundred years.