A basic premise of most infectious diseases (spread by bacteria, viruses and fungi) affecting humans is that subsequent to infection, the body's defence mechanisms (various subtypes of white blood cells) mount an immune response and attempt to kill the infective organism.
When I was driving to school earlier, I was just thinking that if Earth was a living organism, maybe we are the infective agent causing the disease and all the bacteria and viruses that can affect us are its equivalent of an immune system.
This is not a green/earth friendly message or anything btw.. I just thought it was an interesting thought.
Haha.
Luke