smelly carpet

Hi,
My floor carpet is giving me bad headache. Last summer I removed it, washed it with water and detergent, and exposed it to sun. But before drying it was kept folded for almost 12 hours. Since then it has had very nasty smell that does not disappear despite all the efforts I made.
Anybody has the genuine idea to get rid of this smell? ( assuming I want to keep the carpet).
thanks