Genetic variation is the key to evolving, adaptation, and survival.  When conditions change, or a new germ evolves, certain members of a species will have slight variations that allow them to survive better than others.

As factory farming dives head-first toward eliminating as much variation as possible in trying to find the largest, cheapest yields, variation has been sent to the shitter.  The most recenly researched victim is the factory chicken–bred to grow quickly and large[sic].

Some scientists are now working on schemes to try and infuse variant genetic material back into the chicken gene pool.  This will help the chicken withstand devastating germs and bird-flus.  And consumer pressure on sustainable farming techniques is keeping small pockets of chickens further away from the factory pressures that lead to less variation.