The Spanish Flu Pandemic, also known as la Grippe espagnole, or La Pesadilla, was an unusually severe and deadly strain of avian influenza, a viral infectious disease, that killed some 50 million to 100 million people worldwide over about a year in 1918 and 1919. It is thought to have been one of the most deadly pandemics so far in human history. It was caused by the H1N1 type of influenza virus, which is similar to the bird flu of today, mainly H5N1 and H5N2.
Many ballplayers came down with the Spanish Flu in 1918 and some died. For a list of flu victims, go here. During the 1918 flu, ballplayers had sometimes worn masks . Even Babe Ruth had likely caught the 1918 flu and had a 104 degree temperature. When Ruth's team eventually won the 1918 World Series, there was no victory parade in the streets. 
The Allies of World War I called it the "Spanish Flu". This was mainly because the pandemic received greater press attention in Spain than in the rest of the world, because Spain was not involved in the war and there was no wartime censorship. Although the virus certainly did not originate in Spain, the country did have one of the worst early outbreaks of the disease, with some 8 million people infected in May 1918. It was also described as "only the flu" or "the grippe" by public health officials seeking to prevent panic.
The Spanish flu might have contributed to the end of World War I. More people died from the Spanish flu during World War I than were killed in the conflict. (The flu killed at least 20-40 million on both sides; some now believe as high as 100 million). Of course, in contrast with World War II, civilian populations, except those in the immediate vicinity of the front, were largely spared the worst of the war, and the vast majority of its victims were combattants. It is likely that the privations and poor nutrition during the war years that preceded the flu led to the high death toll, as populations were already weakened when the disease hit.