There are conditions called SIBO (Small Intestine Bacterial Overgrowth) and Candida, and medications such as antibiotics and antacids/proton pump inhibitors and the Pill change the balance of the gut flora unless probiotics are introduced to repopulate the bowel. When the gut flora is changed, it allows the bad bacteria and/or yeasts to colonise which cause all sorts of unpleasant symptoms such as IBS, allergies and intolerances, UTIs, etc.
Obviously there's a place for antibiotics which are a wonder drug, but when children are prescribed these frequently, they have an effect. I speak from experience - antibiotics saved my life aged 6, but I have suffered from a range of allergies ever since, and in my childhood no one (or certainly not my parents) understood about probiotics and gut flora.
I learnt this the hard way. 
HRT - Starting for the first time at age 66.
Retiring and living frugally in money from downsizing after years of stress


