As 2006 settled in, Earth had been rendered completely devoid of Transformers, and the rebuilding of Tokyo was underway. However, Galvatron's impact was revealed to have had a much greater effect than anyone could have predicted—the blast caused by his impact had scattered his Unicron-empowered cells throughout the planet's atmosphere. When they came into contact with other animals and objects, from marine life to cars, they fused with them, becoming one of many monstrous creatures referred to as Legion. This catastrophe also created the means to defend the world against the Legion, however—when Galvatron's cells came into contact with human beings, they also become able to fuse with other entities containing his cells by kissing them. With their Autroopers being the perfect candidates for this "ParasiTech" fusion process, the EDC began recruiting and training these "Kiss Players" as combat squads to battle the Legion.

(Source: Transformers wiki)