When candida is growing or dying, it secretes toxins that are very bad for the body. They are hard on the liver and I believe you develop chemical allergies as the body's warning to protect your from further damage.
When I eat well, my MCS is minimal.
EM also helped tremendously, particularly strengthening the spleen meridian and sedating the triple warmer meridian.
I think exposures to other toxins besides candida will also cause it, but candida is a common cause.