As I have pointed out numerous times women in particular get yeast infections after antibiotic therapy because of the shift in vaginal pH to the alkaline side. The normal pH of the vagina is slightly acid, which helps to prevent Candida overgrowth as well as other pathogens. This is because most pathogens cannot survive an acidic environment. The acidity comes from beneficial Lactobacillus bacteria (flora) that produce lactic, acetic and fatty acids. When a woman takes antibiotics the flora are destroyed and the acids not being produced in sufficient quantities to maintain the proper acidic pH. When the pH is shifted to the alkaline side as I pointed out in my earlier post the Candida converts from a mild yeast form in to a more virulent and pathogenic form. Thus the Candida overgrowth.
The pH of the skin and stomach are also normally acidic in large part to protect us from pathogens. I posted a while back evidence of the acidic nature of skin and how it protects us. Also how we get infections and skin breakdown when the skin becomes alkaline.