My mother grew up in a household with nine children and little money. Not much was wasted. Drop a piece of food on the floor and you picked it up, dusted it off, and ate it. This doesn't seem to have done my mother much harm, since she is still around at 97. Her philosophy toward food and life can be summed up in her comment, "You eat a peck of dirt before you die."

Contrast this with the television claim I heard a couple of weeks ago: "Use this product regularly, and you will rid yourself and your house completely of germs and pests."

The term "pest" was not described. It probably didn't include your children's friends. But whatever the definition, the advertisers are kidding themselves and the public by making such extravagant claims. Your house, and you yourself, are swarming with small organisms, whose entry to either place was not invited but whose banishment is a total impossibility.

I have nothing against cleanliness, and certainly no one wants to encourage the presence in your home of the microorganisms that cause cholera, malaria, bubonic plague, and other infectious diseases. Such dangers are, however, very much in the minority. Fatal diseases are also the failures among the household invaders. What's the point of invading a country, if the invasion makes the land uninhabitable? In our case, that amounts to the organism infecting and killing its host. Successful invaders don't kill you, or even make you sick. The most successful ones become so important to you that you could not live without them.

Biologists set up a hierarchy of three types of relationship between living organisms. When one organism does nothing but harm to its host, that's called parasitism. In our case, this includes things like ringworm, pinworms, athlete's foot, ticks, and fleas. All these have become rarer in today's civilized nations, but most parents with children in elementary school have heard the dread words "head lice," and have probably dealt with at least one encounter.

Parasites we can do without. This includes everything from the influenza virus, far too small to see, to the tapeworm that can grow to twenty feet and more inside your small intestine.

Much more common, however, are the creatures that live on and in us and do neither harm nor good. This type of relationship is known to biologists as commensalism. We provide a comfortable home for tiny mites that live in our eyelashes, to others that dine upon cast-off skin fragments, and to a wide variety of bacteria. We are unaware of their presence, and we would have great difficulty ridding ourselves of them. It might even be a bad idea, since we can't be sure that they do not serve some useful function.

And then there is symbiosis, where we and our fellow-traveling organisms are positively good for each other. What would happen if you could rid yourself of all organisms that do not possess the human genetic code?

The answer is simple. You would die, instantly. In every cell of your body are tiny objects called mitochondria. They are responsible for all energy generation, and they are absolutely essential to your continued existence. But they have their own genetic material and they reproduce independently of normal cell reproduction. They are believed to be bacteria, once separate organisms, that long ago entered a symbiotic relationship with humans (and also with every other animal on earth).

If the absence of mitochondria didn't kill you in a heartbeat, you would still die in days. We depend on symbiotic bacteria to help digest our food. Without them, the digestive system would not function and we would starve to death.

"We are not alone." More and more, we realize the truth of that statement. We are covered on the outside and riddled on the inside by hundreds of different kinds of living organisms, and we do not yet understand the way that we all relate to each other. For each, we have to ask, is this parasitism, commensalism, or symbiosis?

Sometimes the answers are surprising. Twenty years ago, gastric ulcers were blamed on diet or stress. Today, we know that the main cause is the presence in the stomach of a particular bacterium known as Helicobacter pylori. Another organism, Chlamydia, is a suspect for coronary disease and hardening of the arteries. A variety of auto-immune diseases may be related to bacterial action.

All these facts encourage a new approach for biologists and physicians: The best way to study humans is not as some pure and isolated life form; rather, each of us should be regarded as a "superorganism." The life-cycles and reproductive patterns of us and all our fellow travelers should be regarded as one big interacting system.

Disgusting, to be lumped in with fleas and mites and digestive bacteria, as a single composite object? I don't think so. In a way it's a comforting thought. We are not alone, and we never will be.

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