The pneumonic plague, which is more likely to be used in connexion with terrorism, is naturally carried by rodents and fleas but can be aerosolized and sprayed from crop dusters. A 1970 World Health organisation assessment asserted that, in a worst case scenario, a dissemination of 50 kg in an aerosol cloud over a city of 5 million could result in 150,000 cases of pneumonic plague, 80,000-100,000 of which would require hospitalization, and 36,000 of which would be expected to die.