Given \(G\) a finite group, and \(S\) a Sylow \(p\)-subgroup of \(G\) (\(p\) prime), we say that two subsets of \(S\) are fused in \(G\) if they are conjugates by a certain element of \(G\). Trying to obtain a more general approach to the study of the \(G\)-fusion property, Puig began the development of the theory of fusion systems, categories with subgroups of \(S\) as objects, and monomorphisms of groups with certain conditions as morphisms. We generalize a classic cohomological characterization of \(p\)-nilpotency of finite groups, due to Wong and Hoechsmann-Roquette-Zassenhaus, to the context of fusion systems, using the notion of classifying space of such systems, developed by Broto, Levi and Oliver, and a specific definition of module over a fusion system.