In the spring of 1918, what was believed to be the initial cause of the flu?

The main outbreak of the Flu of 1918 happened in the fall of that year when soldiers returned home from the trenches of World War I, spreading the disease around Europe and North America. However, the earliest known cases of the disease were documented in military training camps in Kansas earlier that spring. Ever since, scientists and historians have debated the origin of this strain of influenza. Some thought it started in Spain (hence its other name, the Spanish Flu), while others posited France, China, or the American Midwest.
Recent research seems to trace the origins of the epidemic to China. In March of 1917, nearly 100,000 laborers from China were transported to Europe by way of Canada to aid in trench construction on the Western Front. A new theory, backed by evidence taken from medical samples and historical records, points to these laborers unwittingly spreading this disease as they traveled around North America and Europe in the winter and spring of 1918. In fact, an outbreak of a deadly flu was documented in northern China just prior to the earliest cases in North America and Europe.
Whatever the initial causes, crowded military camps and the movement of soldiers and laborers exasperated the spread of this disease that killed as many as 50 million people worldwide.
http://www.nature.com/collections/dpjcqwsqts?error=cookies_not_supported&code=4a7a6514-784e-4d28-8208-960ce639c8e0

https://virus.stanford.edu/uda/