Do you think our body consists of only bones, muscles, nerves and all the other elements which we have studied during our school life? I am sorry, but your books missed on this one thing. Our body is the house to hundreds of micro-organisms. Where few yeast and bacteria are necessary for our body, at the same time they throw some harmful effects too.
Now, you must be thinking of how we can get to know that our body has parasites that are harmful to health. Read the entire article and know it.