Medicine: Stop That Germ!

It's a jungle out there, teeming with hordes of unseen enemies. Bacteria, viruses, fungi and parasites fill the air. They cluster on every surface, from the restaurant table to the living-room sofa. They abound in lakes and in pools, flourish in the soil and disport themselves among the flora and fauna. This menagerie of microscopic organisms, most of them potentially harmful or even lethal, has a favorite target: the human body. In fact, the tantalizing human prey is a walking repository of just the kind of stuff the tiny predators need to survive, thrive and reproduce.

Humans are under constant siege by these voracious adversaries. Germs of every description strive tirelessly to invade the comfortably warm and bountiful body, entering through the skin or by way of the eyes, nose, ears and mouth. Fortunately for man's survival, most of them fail in their assault. They are repelled by the tough barrier of the skin, overcome by the natural pesticides in sweat, saliva and tears, dissolved by stomach acids or trapped in the sticky mucus of the nose or throat before being expelled by a sneeze or a cough. But the organisms are extraordinarily persistent, and some occasionally breach the outer defenses. After entering the bloodstream and tissues, they multiply at an alarming rate and begin destroying vital body cells.

The invaders soon receive a rude shock, for they encounter one of nature's most incredible and complex creations: the human immune system. Inside the body, a trillion highly specialized cells, regulated by dozens of remarkable proteins and honed by hundreds of millions of years of evolution, launch an unending battle against the alien organisms. It is high-pitched biological warfare, orchestrated with such skill and precision that illness in the average human being is relatively rare.

Early-warning cells constantly monitor the bloodstream and tissues for signs of the enemy. With the gusto of Pac-Man, they gobble up anything that is foreign to the body. They envelop dust particles, pollutants, microorganisms and even the debris of battle: remnants of invaders and infected or damaged body cells. Other early warners direct the production of unique killer cells, each designed to attack and destroy a particular type of intruder. Some of the killers, alerted to body cells that have become cancerous, may annihilate these too.

Endowed with such specialized weapons, the properly functioning immune system is a formidable barrier to disease. Even when an infection is severe enough to overcome the system's initial response and cause illness, the immune cells are usually able to regroup, call up reinforcements and eventually rout the invaders. But when the system is weakened by previous illness or advancing age, for example, the body becomes more vulnerable to cancers and a host of infectious diseases. And should the system overreact or go awry, it can cause troublesome allergies and serious disorders called autoimmune diseases.