One of the best, and most popular game modes is returning to the series with Halo 5: Infection. The Infection mode is the official term for what fans once called Zombies. The mode pits two teams against each other, the zombies, or infected, and the humans. The twist is that everyone but one player begins the game as a human, and any time the single zombie character kills someone they switch teams and grow the horde. The winner is the last human standing.
Infection was officially added to the series in Halo 3, after having built up a loyal following among Halo 2 players in custom matches. It has been a staple of the franchise ever since, appearing in Halo Reach and Halo 4 as well. Bungie and 343 have made their own tweaks to the way the game works over the years, but that core concept of an ever growing horde of zombies has remained intact.
The weapons also play a part in building up the human vs. zombie/infected format, with the human team usually wielding shotguns and pistols, while the infected have swords, emulating having to physically bite their victims. This seems to be how Halo 5: Infection will be setup as well, though 343 has really pushed the boundaries in the past, and could do so again.
No release date was given for the Halo 5: Infection update, though many expect it alongside the Memories of Reach update that is planned for release soon. This is mostly based on the Reach style armor seen in the teaser video below.