If you love playing Counter-Strike and are interested in zombies, you should try playing on servers with Zombie Plague Mod or Zombie Escape Mod! 

Zombie Plague is a modification for the Counter-Strike server that completely changes the gameplay, turning it into an intense survival experience of "Humans vs Zombies."

Typically, servers with this modification use maps whose names begin with the prefix "zm_". A list of servers with the Zombie Plague Mod for different game versions can be found at the following links:
CS 1.6 https://www.gs4u.net/en/cs16/search/map-zm_/
CS:S https://www.gs4u.net/en/css/search/map-zm_/
CS:GO https://www.gs4u.net/en/csgo/search/map-zm_/

If you want to play on servers with the Zombie Escape Mod, look for maps whose names begin with the prefix "ze_". A list of servers with the Zombie Escape Mod for different game versions can be found at the following links:
CS 1.6 https://www.gs4u.net/en/cs16/search/map-ze_/
CS:S https://www.gs4u.net/en/css/search/map-ze_/
CS:GO https://www.gs4u.net/en/csgo/search/map-ze_/