The 1918 Spanish Flu Pandemic was one of the worst disease outbreaks ever seen, leading to the death of some 50 million individuals across the globe. It was also the first H1N1 global outbreak — its followup in 2009 was decidedly less potent, at least in part because of advances in medical science since then.
Now researchers have gone back to the months before the 1918 outbreak, and have found evidence that the flu was killing before people even knew what was going on. The scientists examined clinical and pathology records, including available autopsy material, for 68 military personnel who succumbed to influenza or pneumonia between May and October of 1918 — this was months before the pandemic was acknowledged or had peaked.
What they found was that these dead had exactly the same virus as those killed by it later, and that there was no mutation to explain its sudden high mortality rate or ease of transmission. Usually, if a virus suddenly becomes massively more dangerous, there's a change in its form that can be attributed the cause. Instead, the Spanish Flu was exactly the same when it was quietly killing the occasional serviceman, and when it was rampaging around the world in one od the deadliest disasters in human history.
So while we still don't know exactly what caused the disease to go pandemic, this information at least sheds some light on the build-up to it doing so.