One of the main scientific objectives of the recent Fermi mission is unveiling the nature of the unidentified gamma-ray sources (UGSs). Despite the large improvements of Fermi in the gamma-ray source localization with respect to the past gamma-ray missions, about 1/3 of the gamma-ray objects detected still do not have a low energy counterpart associated. Recently, we discovered that blazars, the rarest and the largest known class of gamma-ray sources, can be recognized and separated from other extragalactic sources dominated by thermal emission using the IR colors.

I will present how the Wide-Field Infrared Survey Explorer (WISE) infrared data make possible to identify a distinct region of the IR color-color diagrams where the gamma-ray emitting blazars are separated from the other Galactic and extragalactic sources. This IR non-thermal region of the parameter space, so called WISE Gamma-ray Strip (WGS), it is a powerful new diagnostic tool that can be used to extract new blazar candidates, to identify those of uncertain type and also to search for the blazar-like counterparts of unidentified gamma-ray sources.

First, I will show the relation between the infrared and gamma-ray emission for a selected sample of blazars associated with Fermi sources, for which WISE archival observations are available. Then, for the first time, I will present a low-energy candidate counterpart for 156 out of 313 UGSs analyzed. Finally, I will show the physical origin of the IR - gamma-ray connection and comparing this new association method with the previous ones.