Versus mode is a completely different beast. This mode is aimed at those thirsty for human vs human competition. Four human players play as the Survivors and face off against four human players playing the boss infected. The boss infected players spawn 20 seconds after they are killed in combat, being able to choose their spawn location by walking around as a ghost until they find the perfect ambush spot. This quick spawn time makes Versus mode absolutely brutal to the Survivor team. On top of that, the only place where Survivors are able to respawn is at the next map.
Just like campaign mode, you choose a specific campaign to play through. The first team plays as the Survivors through the first map until they reach the end or until they all die off. The two teams switch spots, so the former Survivors are now acting as the boss infected and the former boss infected are now playing as the Survivors. After the map is completed, scores are tallied based on various factors and the cycle repeats on the next map in the campaign. This continues all the way until the finale map is completed for both sides, at which point the final score is shown and a victor declared.
Versus mode is currently the only mode where human players can choose to play as the boss infected.