Well it probably permeated the infected person so every drop of body fluid is contagious. Every exhale would fill the air with infected moisture from their lungs and mouth. Just watch the smoke exhaled from someone smoking a cigar and see where it goes. If your inside It fills the entire room and outside you can smell it down wind for 100 yards or more. You share and breath their exhaled air. Not only that but skin cells shed all the time. HVAC ducts are filled with skin cells. (sorry to gross you out).