Koro is a poisonous gas that spreads quickly, infecting and weakening those that breathe it in, though leaving them conscious enough to be interrogated. Its effectiveness surpasses the expectations of its own creator, Caesar Clown.
However, extremely strong and resistant victims of the gas can retain some of their strength, to the point of convincing others that they had not been poisoned at all. Additionally, its effects are treatable during the initial 48 hours after exposure, and with the help of Caesar himself, Tony Tony Chopper managed to develop an antidote for the poison.
Under pressure from Tony Tony Chopper and Sanji, Caesar used his Devil Fruit power to create 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 Roko antidote into their bloodstreams.
After five days and five nights of battle between the Beasts Pirates and the Mink Tribe, Jack lost his patience and unleashed Koro to poison the minks, who were left incapacitated and at his mercy.
However, Jack left one day after releasing the weapon, after 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 remaining gas and help produce an antidote, saving the minks.
"Koro" is derived from the word "Korosu", which means "to kill".
↑One Piece Manga and Anime — Vol. 81Chapter 811 (p. 3-7,13-14) and Episode 761, Upon learning of Caesar's creation of the Koro gas, Sanji and Chopper coerce him into neutralizing it and treating its victims.