Recently i downloaded the Infection.7z mod. I tried it out but every time i infected someone the game closed and displayed a message (which i cant show because everytime i open cortex command it doesent show anything but a section of the intro video which freezes) but when it doesnt have the mod it opens a error window. But when it does have the mod it doesnt do anything but show a line of the intro movie which freezes shortley after or it just stays blank and does nothing.

Any help or ideas?

Just reinstall it, it's quick and easy!

Reinstall CC to a fresh directory, and then copy your old settings.ini into it (as well as any fixes, like the ammo.ini fix).
If it says you don't have the mod sometimes and then picks it up other times, it's quite likely to be an issue with the game.