Charles Etzler, aged 12, of Yranan, Colo., aged 12 for his question:

Why do we have some diseases only once?

Very few people have measles, for instance, more than once. The body had a tough fight to conquer this disease. So it made plans to protect itself against another attack, Measles is a virus disease. It happens when our little enemies, the viruses, invade the body. Twenty ­five million viruses can fit comfortably on the head of a pin, Yet, if you have had the measles, you know what a fuss these tiny enemies can make.

Such minute things are hard to imagine, But the wonderful body knows just to set about handling theme It puts up a fight and generally wine. All you do is what the doctor tells you, That helps# but the body carries on the tough struggle. In that struggle It builds up armaments to ‑defend itself against any ‑fight‑by‑that particular‑­virus,

Lets use our imaginations to watch a blow by blow struggle between the body and the invading army of viruses. We couldn't actually see this, For a virus must be magnified 50,000 times to be seen. What's more, it can only carry on its warfare in the living cells of a body,

The invasion starts when the tiny enemies make their way through the walls of the body cells. The little fellows look rather like balls of soft cotton. Once inside, the trouble begins. Right away, the invading army starts to multiply, And viruses can multiply at an amazing rate.

All viruses are parasites, They feed on the precious stuff within your body cell. Since few, if any of them, give anything in return, their gain is your lose. Disease viruses do chemical damage in addition to food snitching. In a day, or even a few hours, after an invasion, countless hordes of new viruses may teem throughout your body,

But the trusty old body is not asleep on the job. It has soldiers to put into the field. In fact, its soldiers are bigger than the enemy soldiers, But enemy soldiers. .But this may be the first time they have met up with this particular kind of virus. If so, they are not armed or fitted to cope with them right away.

The defending soldiers are actually large and complex protein molecules called globulins. Countless numbers of them float all ready in the blood stream, They gain fighting experience as they come into contact with the invaders. This contact changes them. Each globulin changes its shape to fit an enemy virus; It becomes what we call an antibody,

The antibodies keep their new shapes after the enemy has been put to route, After a bout with measles, the antibodies formed to fight the viruses will stay with you for life. There they are, ready to take on any measles virus that thinks it can attack you.

Next time a measles virus gets inside it never gets started. The antibodies are right there and armed, They stop the fight before it begins by blanketing the enemy soldiers. For each antibody is exactly the right size and shape to enfold and smother an enemy soldier.