Now a common disease, Horse Flu infested large part of the major cities of western countries in the late 18th-early 19th century. The virus developed in the slums where coachmen lived with their horses, and quickly spread as that was the only means of proper transportation in the cities and they travelled all around. This disease was rather dangerous to horses, and for the most part humans were unaffected. However, centaurs that caught this disease were terribly affected, lethally.
As the disease was considered dealt by 1810, in the non-magical world, the humans continued with their lives, embracing the use of bicycles and walking as an alternate to the disease infested horses. However, a dangerous mutation hit the cities of Southeast Coast during that time. This mutation, contrary to previous ones, affected humans just a fatally as horses. That particular change resulted in devastation to the middle and lower classes in the large cities, most precisely around the slums. Rock Fortress is one that was hit particularly hard and in the wake of it the largest public hospital of the city came to be, St. Umberta's Hospital. It is estimated that this second outbreak, fatal to humans, resulted nearly millions of casualties - numbers considered appalling by past and present medical societies. The only other case of dangerous outbreak was specific for centaurs in 1931, and affected the First World as centaurs visited Southeast Coast and got infected by the pathogen for the first time, unlike the local centaurs that had developed resistance and were vaccinated against the more deadly mutations.