There are about 1000 different species of microorganisms living inside digestive tract of a healthy human. Those microorganisms are called intestinal microbiom. Those 1000 different species have to be a in a relatively perfect balance in order for you to have a good digestion and good health. And, when one person is extremely healthy, that is usually a sign that his/her intestinal microbiom is in perfect balance.
When a part of the human microbiota, such as the skin flora, gut flora, or vaginal flora, becomes deranged, with normally dominating species underrepresented and normally outcompeted or contained species increasing to fill the void, than you have dysbiosis.
In simple terms, dysbiosis is an imbalance between normally dominating species of microorganisms and normally outcompeted species of microorganisms which usually fill the void when the numbers of the normally dominating species have been shaken.
When people start taking vaccines, Antibiotics , artificial hormones, hormonal birth prevention, other medications, other products of chemical industry like pesticides, fungicides, taste enhancers , food preservatives designed to kill microorganisms, chlorinated water (chlorine is added to water in order to kill microorganisms), pasteurized and sterilized foods, etc, the final possible result of this experiment is a chronic intestinal dysbiosis. You have reached the state of chronic intestinal dysbiosis once your digestive tract is unable to naturally reestablish the perfect balance between normally dominating species and naturally outcompete normally outcompeted species.
In severe cases, chronic dysbiosis may lead to Ulcerative Colitis, Chronis Disease, Colon Cancer, Diverticulitis, and life threatening infections like Clostridium Dificille as well as many other diseases.
It is not my intention to write a book about dysbiosis. If you are interested to learn more about this subject, please follow the next links:
What is increased intestinal permeability ? What is leaky gut?
Healthy intestines are suppose to be partially permeable. Intestinal permeability is a term describing the control of material passing from inside the gastrointestinal tract through the cells lining the gut wall, into the rest of the body. Food that has been properly digested inside our intestines, is suppose to be absorbed by intestines and transported by bloodstream through portal vein directly into your liver. When intestines are working properly, they are absorbing only small molecules, like molecules of water, molecules of digested food like minerals, amino acids (building blocks proteins are made of), fatty acids (building blocks fats are made of) and simple sugars (building blocks complex sugars are made of).
Microorganisms, beside aiding digestion, also help keep your intestines healthy and help keep correct intestinal permeability. Once the balance between microorganisms have been destroyed beyond the ability to be spontaneously repaired, some new microorganisms may take the place previously occupied by the good ones, like fungal form of Candida albicans, and toxins produced by those "bad" microorganisms may damage intestines even more, more than they have already been damaged by, for example, Antibiotics and other products of chemical industry. That is a very poor and very short story how you develop increased intestinal permeability. Books are written on this subject, if you are really interested to learn more.