There are certain cells in the blood whose surfaces have holesall over that exactly fit every kind of bacteria and virus that youare immune to. The germs fall into those holes, are captured, andcannot cause infection.Now, along comes a bunch of a certain virus that you are notimmune to - they do not fit any holes in those certain cells. Thecertain cells try to grab the new viruses, but can't, so theyrelease a tiny bit of chemical that travels in the bloodstream tothe brain, causing the brain to release a tiny bit of chemical intothe bloodstream that starts up the disease-fighting mechanisms ofyour body.