But when our body tissues become excessively acidic, these microbes bloom into life and take on a different purpose in the body, a role which is completely logical if you concentrate on it : they’re there to rot dead tissue and recycle it into the soil of the earth.

That’s what Dr. Young means by only 1 illness. When our tissues become very acidic, this is the condition that all organisms go into just before death, so the microbes within get the signal that we are dying and go into action to rot us. The symptoms would begin at the weakest link in our bodies, so it may begin with any of the illnesses humans get. But in fact there’s only the one illness : over acidification that tells the microbes we are prepared to die and rot.