If you already own a legitimate or clean non-Steam version of CS 1.6 (folder size ~200MB), you can download just the mod files.
While searching for "Zombie Plague," you might encounter Zombie Escape (ZE). Do not confuse them. counter strike 16 zombie plague mod download link
It is important to note that you cannot simply click a single "play" button. Zombie Plague requires a base installation of Counter-Strike 1.6 and the mod files themselves. If you already own a legitimate or clean