Many cars that have perfectly good engines are scrapped. Some have been involved in accidents. Others have rusted so badly in places that they can no longer be driven.

 

In this way, human beings are like automobiles. Much of a person's body, in old age, may remain in sound, working order. But if a serious illness damages just one essential organ, such as the heart or lungs, that person may die.

 

One way to extend human life, then, is to lessen the threat of as many dangerous illnesses as possible. In the case of a car, the body work can be coated with special paints to make it more resistant to rust. Similarly, it is possible to protect people against certain types of diseases (see Figure 1).

 

The threat of rust to a car can be reduced further by regular washing off dirt and salt. In the same way, a sensible diet, plenty of sleep, and frequent exercise help to prevent illness in human beings. Finally, if a car does begin to rust, the affected parts can be treated or even replaced. This is also true of a sick person. Doctors can now offer an immense variety of treatments for the things that can go wrong with our bodies.

 

Body Wars

Many illnesses are caused by germs that enter the body from its surroundings. Germs are living particles too small to be seen without a powerful microscope. Some are so tiny that more than 200 million of them would fit on the period at the end of this sentence. Most of harmless. In fact, huge numbers of germs normally live inside us and on the surface of our skin without affecting us in any way.

 

But some germs do make us ill. Once inside the body, they begin to break into healthy cells and use important substances that our cells produce. Fortunately, the body can fight back.

 

Special cells known as white blood cells constantly patrol our bloodstreams, seeking out and destroying invaders. These invaders are particles that the white blood cells do not recognize as belonging to the body. They include germs and specks of non-living substances such as dust and soot which we may breathe in or swallow (see Figures 2, 3, and 4).

 

Different types of white blood cells perform different jobs. One, type, called macrophages, simply roams around our blood, devouring anything that does not seem as though it belongs there. If such a cell meets an invading germ, it immediately sends out a chemical message for more help. Another kind of white blood cell, called a helper T cell, quickly arrives to identify the germ. Then the helper T cell rushes off to give instructions for the making of antibodies and killer T cells.

 

Antibodies are substances produced by the body to attack specific germs. One type of antibody is only effective against one type of germ, If a supply of the correct antibody is already available, the invading germs can be killed before they have time to do any damage. But if the germs are a kind that the body has not had to fight before, new antibodies have to be made. While this is happening, the invading germs can break into healthy body cells and start to multiply. Then, when the battle between the body's defenses and the infection does take place, it is on a much larger scale.

 

Killer T cells are another type of white blood cell. They are produced by the body to destroy cells that have been damaged by germs. If killer T cells do not carry out their mission, the infection continues to spread.

 

The whole system by which white blood cells are organized to defend the body against infection is known as the immune system. Every minute of every day, the immune system fights against hosts of invading particles that threaten our health. Some of these unwelcome guests, such as the germs responsible for colds and flu, may only cause discomfort. Other germs that get inside our bodies, though, can be life-threatening.

 

How Our Bodies are Made The human body is like a complicated machine made of many interconnected parts (see Figure 5). At the smallest scale, the body is composed of about 100 trillion tiny living cells. There are different kinds of cells, each with its own task to do. For example, nerve cells carry messages of sensations and pain to the brain.   Groups of similar cells are organized together to make tissues, such as skin and muscle. In turn, various types of tissues combine to form organs. Among the body's major organs are the heart, brain, stomach, and kidneys. Finally, groups of organs and other body parts work together as organ systems. The digestive system, for example, involves such organs as the stomach, intestines, and liver. Another system, the immune system, is a complicated mass of organs and chemicals whose job is to combat infection and disease. The heart and veins make up the circulatory (blood) system, which keeps the blood flowing around the body.

 

The Deadly Invaders

It seems strange that some of the tiniest creatures on Earth present one of the biggest threats to a longer and healthier life. A germ that is too small to be seen with an ordinary microscope can kill a strong, healthy adult. Enough germs to destroy the entire population of New York can fit comfortably on a pinhead. These tiny creatures multiply so fast that a single germ at breakfast time can become an army of more than 100 billion by late evening.

 

The word germ describes a wide variety of tiny living things. One group of germs, bacteria, are plants consisting of a single cell. They use substances from other cells to help them multiply. Some bacteria are useful to the human body. For instance, a chemical produced by one type helps blood to clot. Other bacteria, however, cause infection and disease (see Figure 6).

 

A different kind of germ, called a virus, is even smaller than bacteria. It is so small and simple that, unlike bacteria, it cannot make copies of itself at all. Instead, when a virus has broken into a cell, it releases a set of chemical instructions. These instructions take over from the ones already present inside the cell and tell the cell to build thousands of copies of the virus. Eventually, the new virus particles burst out of the cell and invade other parts of the body (see Figure 7).

 

Some of the most deadly human diseases result from infections by bacteria and viruses. Typhoid fever, cholera, tuberculosis (TB), and bubonic plague are just a few of the life-threatening illnesses caused by bacteria. Years ago, there was no treatment for these illnesses. Outbreaks occurred in which millions of people died. Today, though, scientists have developed a range of substances that will kill disease-causing bacteria. These substances are known as antibiotics. The first to be discovered, and the best known, is penicillin.

 

Sadly, it is much harder to treat patients who have diseases caused by deadly viruses such as AIDS. So far, no drug has been developed that will kill a virus once it has invaded a person's body. On the other hand, people can be protected from future attacks by some viruses through immunization. This can be done by introducing a vaccine into the body, usually through injection with a needle. The vaccine contains either dead germs of the disease or living germs treated so that they do not cause illnesses. To defend itself against the vaccine, the body produces antibodies. Then, if the person later comes into contact with this virus, his or her immune system recognizes it and makes the correct antibodies without delay.

 

Vaccines exist today for many of the most dangerous, common viruses. In addition, the broad range of antibiotics available means that infections caused by bacteria can usually be cured. Partly for these reasons, the average life span of men and women in developed countries has risen steadily over the last century. In the United States, for instance, a typical person born in 1890 could expect to live less than 44 years. By 1990, the expected life span for newborns had increased to 76.1 years for males and 83.4 years for females.

 

In the future, even more effective ways to fight life-threatening viruses and bacteria will be developed. Some of these will involve making drugs inside living cells that have been engineered to produce certain substances. These new drugs will be designed to lock onto and destroy particular invading germs without harming the rest of the body.

 

It may be possible to gain complete control of certain diseases. This has already happened in the case of smallpox, a disease similar to chicken pox but much more severe. Health workers won the battle against the smallpox virus in the late 1970s through a worldwide program of immunization and by isolating victims until they had recovered.

 

Cancer and Heart Disease

Not all serious illnesses, though, result from germs that invade the body. Two of the leading causes of death in the Western world are cancer and heart disease. Neither of these can be passed on from one person to another.

 

According to some scientists, cancer may start when just one cell among the 100 trillion or so in a person's body develops a fault. Inside a normal cell are chemical instructions that cause the cell to divide in an orderly, controlled way. This is the body's means of replacing cells that have worn out and died. But once in a while, something may slightly alter the instructions within a cell. As a result, the cell starts to divide in an uncontrolled way. It becomes cancerous (see Figure 8).

 

Cancer cells have no regular shape or size. As they grow, they destroy neighboring healthy cells and use their contents to create more cancerous ones. In this way, they starve the body of blood and important chemicals that it needs to stay healthy.

 

Cancer can begin in almost any part of the body and spread slowly or quickly to other areas. It may be caused by swallowing or breathing certain substances, such as tobacco smoke, over a long period of time. But in most cases, it is difficult to tell how cancer starts. It is possible that an invading virus alters the chemical plans inside a cell, causing the cell to grow uncontrollably. However, the disease may only become obvious years later.

 

Just as cancer has no single cause, it has no single cure. Doctors can apply three main methods in the battle against cancer. They may use powerful drugs or beams of radiation to destroy the cancer cells. These two methods, though, may also kill healthy cells. Doctors may also choose to operate on the patient to remove the affected part of the body.

 

In the future, scientists hope to identify the exact chemical differences between an ordinary cell and a cancerous one. Then, new drugs may be developed that will destroy cancer cells but leave healthy cells unharmed.

 

The other leading cause of death in Western countries is heart disease. New advances in medicine may also greatly reduce the threat to this most vital organ in the human body. Many people die in old age because of problems with their heart or blood circulation. The heart is a pump consisting mostly of muscles that work automatically. As we grow older, these muscles weaken. Between the ages of 25 and 65, the amount of blood pumped by the heart decreases by about 13 percent. By itself, this drop in heart muscle-power is not very important. The problem is that the condition of our blood vessels gets worse at the same time.

 

The large blood vessels that carry blood away from the heart are called arteries. In most young people the arteries are clean and wide open. This makes it easy for the heart to pump blood through them. But as we grow older, a fatty, porridge-like substance may collect on the inside walls of the arteries. As the opening of the artery narrows, the heart has to work much harder to push through the same amount of blood. Yet, the heart becomes weaker with age. At some point this may mean that too little blood is able to reach some vital part of the person's body, such as the brain. Then unless help comes quickly, the person is likely to die.

 

Bones also suffer from aging. The most important building-block of bones is calcium. But as people get older, bones lose some of their calcium, making them easier to break. In women, especially, osteoporosis can be a major problem. This is a condition in which hollow spaces develop inside the bones. It makes the bones weak and brittle. For a person with osteoporosis, even a minor fall can result in broken bones.

 

Finally, old age weakens the most important part of the human body, the brain. Every 10 years after the age of 20, about 10 percent of the brain cells controlling higher thought shrivel up and die. This is not as serious as it sounds because the brain has many spare cells that can take over from those that are lost. Still, the loss of brain cells does cause gradually weakening of our memory and power of concentration as we grow older.

 

Someday, it may be possible to slow or halt all of the changes that affect us as we age. However, this will not help us live much longer as long as there are many diseases from which we can die early. It is these more immediate threats to a long and healthy life that need to be dealt with first.

 

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