Maybe a bit off-topic, I wholeheartedly agree with your teacher, but even if a historical event has several causes, it's very common for there to be a single trigger that starts the whole thing. Everybody "knows" that the first world war started with the assasination of the archduke Ferdinand, but the causes were many (the alliance between France and Russia, between Germany and Austro-Hungary, the problems in the Balkans between Russia, the Ottoman empire and Austro-Hungary...)
As for your question, it probably has happened a few times (avian flu a few years ago?), but medicine is much better now and, on average, the population is healthier. Just these two facts alone make it much more difficult (but not impossible) for something like that from happening again.
Another off-topic, but I find it weird that it's called the Spanish flu when, apparently, it started in France. Spain was not part of WWI and didn't have wartime censorship at the time, so it was the first place that started reporting cases of it, but it didn't originate there.