How did the movement of soldiers during World War I contribute to the global influenza (flu) pandemic of 1918 to 1919, which killed millions of people?
A. Eating unfamiliar foods in foreign countries made soldiers sick to their stomach.
B. Soldiers infected in one country carried the infection to other countries.
C. The use of fossil fuels for ship and railroad transportation polluted the air.
D. Soldiers who needed to work together did not know one another's languages.

The influenza epidemic of 1918 (also known as the great epidemic of influenza or the Spanish flu) was an influenza pandemic of unusual severity. Unlike other epidemics of influenza that affect children and the elderly, many of its victims were young and healthy adults, and animals, including dogs and cats. It is considered the most devastating pandemic in human history, since in just one year it killed between 40 and 100 million people. This death toll, which included high infant mortality, is considered one of the examples of mortality crises.

In the United States the disease was observed for the first time in Fort Riley (Kansas) on March 4, 1918, although already in the fall of 1917 there had been a first wave herald in at least fourteen military camps. One researcher claims that the disease appeared in Haskell County in April 1918. And, sometime in the summer of that year, this virus suffered a mutation or group of mutations that transformed it into a lethal infectious agent; the first confirmed case of the mutation occurred on August 22, 1918 in Brest, the French port through which half of the Allied American troops entered in the First World War. It received the name of Spanish flu because the pandemic received more attention from the press in Spain than in the rest of Europe, since that country did not get involved in the war and therefore did not censor information about the disease.