From The TimeSplitters Wiki

Virus is an Arcade mode in which one player starts off as the "virus" and other players must run away from them. It is available on almost every level, it is not available to play in "Siberia" in TimeSplitters: Future Perfect due to the enormous open space the map is based around.

Aim

The aim of virus is to be the last person to become "infected". You become infected if an infected character touches you, and you remain infected until the end of the match. A human controlled player can never start off as the virus, and so will always be running from them. In TimeSplitters: Future Perfect, infected characters gain a slight speed boost making it impossible to escape if you just run in a straight line. The match can be set with a time limit, or you could just play until there is only one winner.

Advice

To get the most out of this game, create a custom weapon set consisting entirely of "unarmed". This will leave you unable to simply shoot persuing viruses and make the game much more challenging. It is also best to play on a small or a medium map, since there is more chance of running into an infected character. Levels with small corridors are also perfect for playing virus in.

In TimeSplitters: Future Perfect, the Virus mode also emits a distinct clicking sound once you start playing that certain game. This clicking sound alerts you of approaching enemies, the more frantic the clicking, the closer you are to an "infected" enemy, and vice versa. This technique only applies to the 1st player, and not others in the Multiplayer modes.