September 1918 to November 11, 1918 was one of the deadliest periods in history due to the overlap of the biggest battles in Europe and the spike of global flu pandemic in the exact same period.
The number of total persons killed in WWI varies greatly depending on what is taken into account. In terms of deaths, numbers approaching 15 or 20 million are often quoted. And in terms of casualties, which includes wounded and disable, numbers approaching 40 million are quoted.
The flu pandemic infected an estimated 500 million people, and 50 million deaths is often quoted, although some argue it was well beyond that.