How regular is regular, they don’t say. It might be passed around through every hundredth person or something like that. Also what does it take, intimate contact or the kind of casual contagion that spreads colds? Getting the virus earlier and having it overcome and learned by the immune system might also interfere with it being available when it would do good to a cancer patient.
Don't you think that that is where inundating the body with a large enough quantity of it would work as the immune system can only handle just so much at a time?