One of these days, someone's going to genetically engineer a virus that spreads easily to infect as many people as possible, but stay dormant for a few years while in this phase, then suddenly it'll turn lethal. Someone from the future might use a time machine to try to come back and gain information about the virus, and then even try to stop its spread once he finds the rogue researcher releasing it at airports, but then will tragically find out that you can't change the timeline, even if you can travel back in time.
