The flooding is the biggest natural disaster in history? The historians at the UN would definitely be clobbered if they competed on "Jeopardy," or perhaps history goes back only a few decades these days.
The Black Death killed half the population of Europe, estimated at 75 million, when the total population was a fraction of what it is today. There were other disasters of smaller size, but far worse than the flooding, like the post-WWI flu pandemic that killed 3% of the world's population.