Narrator:       This is Science Today. Since the beginning of industrialization, allergic and autoimmune diseases have been on the rise. Many scientists believe an explanation for this is the "Hygiene Hypothesis," which proposes that a lack of early exposure to pathogens increases the risk of developing allergic and autoimmune disorders later in life. Researchers at the University of California, Berkeley recently discovered that pathogens were the single most influential environmental factor on shaping genetic diversity in humans.

Fumagalli:      We believe that this is evidence supporting the so-called hygiene hypothesis.

Narrator:        Study leader Matteo Fumagalli and his team also discovered that many of the genes shaped by pathogen interaction were also associated with autoimmune disease.

Fumagalli:      Our immune system has been shaped by an environment full of pathogens. So, now we live in a different environment with less pathogens. So this could be one of the causes of autoimmune disease.

Narrator:        For Science Today, I'm Larissa Branin.