Epigenetics
Epigenetics explains that the environment can change the expression of our DNA, by turning genes on and off. Environment qualifies everything that is around us as humans: what we eat, what we see, what we listen to, what colors we see, everything that is received on a sensorial level. This is why stress of any unpleasant experience can make us feel ill, or cause long term illness; as chemicals are released into our blood stream based on our environmental factors.