They are just like microbes. Only difference between these people and the uni-celled organisms is that it's easier to spot them with their pensive eyebrows, frowned chins, and a smile-less "sky is going to fall on my head any minute" look on their faces.
Try opening up a small talk about a rainbow, they'll be thinking about muddy rains. If you say flowers, they're already sneezing with fear of pollen. While me and you think of beautiful waterfalls, these people would be scared to death of the falling rocks. Ok, we can collectively sigh at these dreary word associations, but it goes on and on.....Chocolates gives headaches, travelling would definitely involve airplane crashes, a walk on the seaside will lead to a topic on tsunamis. By the way, don't even think of cracking silly jokes to cheer up because they'll start gagging over them. Pathetic, huh?
So, in a honest appeal to restore sanity to the world, let's ask these paranoids to please stop worrying and start living. Right now. And, give the rest of us some peace on earth.