I understand that. But why can vapor droplets go from subject to soil to subject, But not subject to subject. Your reply gave me 2 thoughts. Mycobacterium from subject must incubate for some time, as in a culture or in soil, before it can affect humans. That's one idea. Or maybe it is the size. From human to human the bacteria may not be viable in the new subject. Must undergo some change. But then I think.... If it's not viable in new subject as is, then why viable in initial subject? Again just trying to figure the logic