Koro

Koro is a chemical gas weapon invented by Caesar Clown and used by the Beasts Pirates. It's described as a mass slaughter weapon.

Effects
Koro is a poisonous gas that spreads quickly, intoxicating and weakening those that breathe it in. It's effectiveness was enough to surpass the expectations of its own creator, Caesar Clown.

However, despite its strength, its effects are treatable during the initial 48 hours after exposure; past this deadline it becomes untreatable.

Tony Tony Chopper was also able to develop an antidote for the gas' poison with the help of Caesar Clown.

Neutralization
Ordered by Tony Tony Chopper and Sanji, Caesar created a gas to completely neutralize Koro's effects and named it "Roko". Roko managed to eradicate Koro's residue, but did not cure those who were poisoned by it. Instead, the poison had to be expelled from their bodies by injecting a liquid form of the antidote into their bloodstreams.

Zou Arc
After a 5 days and 5 nights battle between the Beasts Pirates and the Minks, Jack apparently lost patience, and he brought out the Koro gas weapon to instantly defeat his adversaries. Being poisoned, none of the fighters were able to resist any further, and fell into the mercy of Jack.

However, Jack left one day after releasing the weapon, in which the Straw Hat Pirates arrived, bringing along the weapon's inventor, Caesar Clown. Revealing he was the inventor of such a weapon, Sanji and Chopper beat him up before forcing him to neutralize the remnant gas and help produce an antidote, much to Caesar's reluctance.

Trivia

 * "Koro" is derived from the word "Korosu", which means "Kill".