Microbes have been among us since time began–back when a chemical soup stirred up the first rumblings of life– yet amazingly our knowledge of them is recent.
Probiotics and their considerable powers were revealed when the Roman naturalist Pliny the Elder used fermented milk to treat intestinal problems. 
It was a natural remedy but no one knew why it helped.
In the late 1800s, scientist Ilya Mechnikov observed that rural people in Bulgaria, in spite of extreme poverty and harsh climates, lived to very old ages; they had average life spans that stretched longer than those in wealthy European populations. Their diets, Mechnikov noted, were rich in yogurt and other fermented milk products.
Luckily, tools of modern science including the microscope gave bacteria involved in fermentation their long overdue close-up. Mechnikov’s work at the Pasteur Institute in Paris reinforced his theories on the benefits of the lactic acid bacteria produced by fermentation. Mechnikov and his colleagues were so convinced that they began drinking sour milk, thereby introducing the modern probiotic, which means “for life.” Around the same time, another important discovery was made at the Pasteur Institute by Henri Tissier who isolated bifidobacterium from the gut flora of breast-fed infants. These bacteria, he observed, could lessen diarrhea in babies.
For the next several decades, exploration into probiotics moved slowly. But as a new century approached, probiotics research with randomized, controlled clinical studies soared, showing both preventive and therapeutic roles for probiotics in health.
The World Health Organization’s (WHO) definition of probiotics, issued in 2001 is as follows: “live microorganisms which when administered in adequate amounts confer a beneficial health effect on the host.”
There is still much to learn: for instance, one probiotic may not perform the same in different people due to different microbiota communities, immune status, and genetic makeup. It is an exciting time in microbiology as scientists work to reveal the specific talents in each microbe inside of us.
The rest is history.