Influenza might not seem as awful as many of the other viruses on this list, but you really need to think about how impactful it is. The flu remains highly contagious, and mutates on a regular basis. It’s so widespread, most people underestimate how many people it kills.
Every flu season, an average of 500,000 people will die as a result of the flu. Take that in, and you’ll understand why it’s such a terrible virus.