By considering the epigraphs of conjugate functions, we extend the Fenchel duality, applicable to a (possibly infinite) family of proper lower semicontinuous convex functions on a Banach space. Applications are given in providing fuzzy KKT conditions for semi-infinite programming. Key words. Fenchel duality, epigraph, KKT conditions, semi-infinite programming. AMS subject classifications. Primary, 90C34; 90C25 Secondary, 52A07; 41A29; 90C46