It's caused by your body reaching an abnormally low temperature. That's why it's called a "cold". The low temperature causes your boogers to freeze, clogging up your nose, and over time they melt into drippy snot. Your throat gets scratchy because it literally frosts over with ice.
That's why you're supposed to eat chicken soup when you have a cold -- the hot soup warms you back up again. Although it doesn't really have to be chicken soup; any kind of hot soup will do. Chicken soup gets the most press because of lobbying from the chicken-farming industry.
So why are colds contagious? Well, that's a myth, really. Lots of people often catch a cold at the same time because the weather outside is cold, and they all live in the same place. You might look at it and think it's being spread around from one person to another, but really they're independent cases that have a shared cause. (This is why you're still going to school this week, Calvin.)
It's general knowledge that there's no cure for the common cold, and that's because in order to cure the common cold, you'd have to stop cold weather. But as it turns out, scientists have actually been making progress in this regard! That's what global warming is all about: a concerted effort by doctors around the world, trying to cure the common cold.