How did World War 1 help intensify the spread of the Spanish Flu?

The great influenza pandemic of 1918-19, often called the Spanish flu, caused about 50 million deaths worldwide; far more than the deaths from combat casualties in the World War One (1914-18). In fact, it may have killed between 3% and 6% of the global population.

At the time, pandemic influenza was new to the world and only people exposed to milder forms of influenza in earlier flu seasons (usually winter) had partial protection against this more virulent form of the virus. And since it happened in the pre-antibiotic era, heavily infected patients were likely to die from viral pneumonia and complicating bacterial infections.