What is responsible for the deaths of at least 30 million people in the 20th centery?

Influenza, or 'flu for short. After the 1st world war, 'flu killed more people than the war did. It was a terrible pandemic.

You can read about it on Stanford University's website here : http://virus.stanford.edu/uda/

Here is a snippet:

The influenza pandemic of 1918-1919 killed more people than the Great War, known today as World War I (WWI), at somewhere between 20 and 40 million people. It has been cited as the most devastating epidemic in recorded world history. More people died of influenza in a single year than in four-years of the Black Death Bubonic Plague from 1347 to 1351. Known as "Spanish Flu" or "La Grippe" the influenza of 1918-1919 was a global disaster.