What is the purpose of colds and flu?

As you may be able to tell from the title I'm tucked up in bed feeling very sorry for myself suffering with a cold (or possibly man flu). It's been a while since I've been ill like this and I can't say I'm happy about it.

Just what purpose is it meant to achieve?

Someone from a certain health profession told me once that it's the bodies way of telling you it needs a rest. But when you get over a cold or flu you hardly feel rested! You generally feel worn out and grumpy.

I jut can't understand the point.

In an age where we can travel to the moon and create nuclear power, surely we can eradicate the common cold! Okay, if I was in a more agreeable mood I may consider the fact that the human body is as complicated an issue as space travel , but I'm not, so I'm just going to rail at the injustice of lying in bed with a headache in the 21st century.

Okay rant over, I'm going to make a lemon and honey tea.