From Halopedia, the Halo wiki

The Repellant medal is awarded in the Halo: Spartan Assault Flood missions for not being the first player to be infected. It is represented by a blue shield with a backward "no symbol" and two spraying aerosol cans.

Players are infected by having a large infection form latch on to their backs. Once one co-op partner is infected, the other player will receive the medal.