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Name: alison
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why do you never normally get the same diseases twice?

In order to kill invading organisms, your body's soldier cells must first recognize the invaders as bad guys. But this is not easy. After all, your body contains a large variety of its own cells -- how can the soldier cells distinguish between a cell from your own body and a foreigner? If YOU saw a couple thousand of your own cells, of various types, mixed up with a handful of invader cells, you would be hard pressed to see the difference.

Because of this difficulty, it takes a little bit of time for the body to recognize invaders it's never experienced before. I don't know exactly why -- perhaps an immunologist would. Hence the first time you are exposed to invaders, your body cannot mount an effective defense for a little while. You get sick as the invaders multiply and release various chemical poisons. Finally your army recognizes the invaders as bad guys and soldier cells specially equipped to kill them start multiplying. Since the soldier cells have the home field advantage, they multiply faster, and eventually your army wipes out the invaders and you get better.

Your army is not stupid. After the invaders are wiped out, a few specialist cells are kept around who can instantly recognize the same bad guys, if they turn up again, and sound the alarm. If the invaders return, the soldiers can start multiplying immediately. Since the bad guys don't have a head start this time, they get wiped out almost immediately, and you don't begin to feel sick. (In other words, you are being infected all the time with the same bugs, but after the first time you wipe them out so fast that you experience no symptoms.)

It's not enough to recognize invading cells, however. Consider viruses. A virus is just a loose DNA ``program'' that finds its way into the ``computer'' (nucleus) of one of your cells. It instructs the cell to make more viruses instead of doing what it's supposed to do. How can your army fight this? Well, it must recognize which of your own cells have been taken over by the enemy and reprogrammed and kill them. This, again, takes time the first time, and then it becomes faster. So you become immune to viruses, too.

Viruses are very small and simple, however, and so they can change easily. Every time they change a little bit, they seem like completely new invaders to your army, and it must learn to destroy them all over again. The cold and influenza viruses, for example, change every year or so. You keep getting infected, sick, and then immune to them, but there's always a new one next year.

The AIDS virus is particularly fiendish this way. It changes very fast once it infects a person. The body keeps recognizing the new variants and wiping them out, but eventually, after many years of stalemate, the virus finds one particular form that the body can't recognize, and that's when full-blown AIDS sets in.

What happens if the body makes a mistake and attacks healthy cells instead of virus-reprogrammed cells? Then you have an ``autoimmune'' disease, such as arthritis, diabetes I, inflammatory bowel disease, multiple sclerosis, or lupus.

What happens if the body fails to attack cells that have been reprogrammed by a virus, chemical, or accident? Cancer.

So you see, your army walks a fine line between too hair-trigger of a response (autoimmune disease) and too lax a watch (cancer).

Incidentally, the invaders do not ``want'' to wipe you out. Indeed, if they do so, they lose their lives, too. A disease that kills quickly is, ipso facto, an evolutionary failure, like a dinosaur that eats up all its food supply and then starves to death. What happens is that bacteria try to colonize you, to live happily inside you. But they don't know how. They release waste products that accidentally poison you, and then they get noticed and wiped out by your army. Successful bacteria, like E. coli, learn to coexist peacefully inside you. So disease can be described more accurately as failed inter-species negotiation than as warfare.


Your have a wonderful immune system that remembers the antigens (foreign protein identifiers) of diseases. When you are invaded a second time, your body can respond quickly from its memory cells to prohibit the disease from becoming infested. You may have felt a cold coming on with a sore throat starting only to wake up the next day with no symptoms of any kind. This may have been your immune system quickly dealing with the invasion.

The complexity of this process requires some study on your part if you are interested. A good high school biology textbook will do the trick in educating you further.

Steve Sample

Anything that enters you body that your immune system doesn't recognize is called an antigen. Your body has an army of cells whose job it is is to look for and destroy any antigens that enter. Some of these cells release proteins called antibodies. When an antigen is seen a specific antibody is designed especially for it. It takes a while for these antibodies to start to work and that's why it takes time for you to get better. At the same time a group of cells is created that is designed to remember that antigen if it ever comes back. They patrol your body and are on the lookout for that antigen and if they see it, they snatch it up so fast that your body doesn't even know it was there. So you don't get the same diseases twice.


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