The influenza virus that emerged in 1918 was more deadly than was typical for the disease. Because of the Great War, the virus was carried to every corner of the world, including into populations of human beings who had never known the disease before. The death toll was staggering. This epidemic was the deadliest in human history, in terms of absolute number of persons killed.
Influenza has plagued the human race for some 12,000 years. It is caused by a virus, an infectious agent barely understood in 1918.
Once Bulgaria quit the war, the dominoes began to fall. By early October, both Germany and Austria were in diplomatic exchanges with the US over peace terms. When news of this became public, both of those governments experienced domestic political collapse.