Koo syndrome is caused by skin-to-skin contact with other recipents. It is not a lethal and serious disease, but possible side effects are the excessive use of "koo" for other words.
An example of the use of "koo" between two Koo syndrome recipents:
Creeper_Corrupt: Koo join training or demote
(Random dude): No stfu koo im a lazy ass
Creeper: Do you want the "Degradierungzeit?!"
Dude: Oh s### imma join training now no demot pls papa Creep