ShootMania Infection is a custom game mode for the arcade shooter created by Nadeo. It features classic infection gameplay as you might know it from other games like Teeworlds or some Call of Duty games.
The mode features two teams: the survivors and the infected. The goal of the survivors is to stay alive for a specified amount of time without getting caught by an infected. The infected start with only a single player and cannot shoot (except the first player as long as he is alone). They infect other players by touching them making them join their team. They win when every player has been infected.
Unfortunately the game is nearly dead by now and I shut down my server due to this, but you can still find many videos on YouTube.