Why does my a/c have a terrible, unpleasant smell? on 2011 Toyota Corolla

When the a/c is on, it has a bad smell. I have reported this twice to the dealership since purchasing my vehichle. They have shown me that my air filter is clean. They don't know what the cause is. I live in FL and I feel that a/c is a necessity but the smell is so bad, that I don't run the a/c often.

Asked by for the 2011 Toyota Corolla
keep going back till they do SOMETHING about it. poss dead animal inside or bacteria growing