Vaccination! I don' t know your age but I am guessing if your old enough to be asking this question, then you had vaccinations as a baby, teen, adult. And vaccinations from the time your were born contained mercury. IN some cases infants receive 50-100 times the EPA safety limit for exposure in one day. Since mercury is cumulative, every little bit adds up over the years until you are sick.
Take a peak at your vaccine records. Also, do you have any tattoos. Do you get flu shots?