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, barring "Siberia" in TimeSplitters: Future Perfect due to the enormous open space the map is based around.
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 can just play until there is only one winner.
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 pursuing 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 radation clicking sounds 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 Player One, and not others in the multiplayer modes. Also, Character Abilities are lost so all the characters are evenly matched to run away from the Virus. The weapon of choice is the Mag-Charger because it can shoot through walls, making it easier to find and kill the virus. The mag-charger is looked down upon by users however, especially in virus mode. Using the mag-charger almost ALWAYS results in the infected characters collaborating in an insane bunrush upon the offending character, frequently killing him after infecting to rub it in further. It should be noted that killing the virus causes it to spawn again and may cause your route of escape to become blocked.
A common strategy attempted by novices is to commit suicide if a virus is on their tail. This however, was predicted by the Free Radical staff, and if someone is to die by their own hand, they are respawned as an infected. This makes certain levels, such as Zepellin, even more fun to play virus in due to the possibility of trapdoors causing you to plummet to your doom, which the game interprets as suicide.