In contrast to infectious, or extrinsic, diseases, noninfectious, or intrinsic, conditions are neither contagious nor communicable. They arise from inside the body as a result of hereditary conditions or other causes, such as dietary deficiencies. Although infectious forms historically have been the most life-threatening varieties of disease and remain so even today in much of the world, non-infectious disease is a far more serious concern in industrialized nations, such as the United States. Some categories of intrinsic diseases include stress-related, dietary, and autoimmune conditions as well as diseases of unknown origin. Additionally, there are several other categories we examine in this essay: hereditary diseases, such as hemophilia; glandular conditions, of which diabetes mellitus is a powerful example; and cancer, of which there are approximately 100 different varieties.
The world has long suffered under the threat of infectious diseases, some of which include smallpox, chicken pox, cholera, malaria, diphtheria, scarlet fever, influenza, polio, pneumonia, and even the common cold. Except for the last one, which is rarely fatal, such conditions have racked up a considerable death toll. For centuries people attributed these diseases to all manner of false causes, ranging from divine curses to an imbalance of bodily fluids. Only with the development of the microscope in the 1600s did scientists begin to identify the real cause behind most infectious diseases: bacteria, viruses, and other parasites. Infectious diseases, discussed elsewhere in this book (see Infection, Infectious Diseases, and Parasites and Parasitology), continue to pose a threat to life in underdeveloped nations. In North America, Europe, the emerging capitalist democracies of eastern Asia, and a handful of other materially and technologically advanced lands, however, infectious diseases have taken a back seat to noninfectious ones as well as to other threats.
In the United States today, the four leading causes of death (see list in Disease) are noninfectious conditions: heart disease, cancer, stroke, and chronic obstructive lung diseases. The first of these conditions is discussed in Disease, and the second is examined later in this essay. Stroke, which occurs when a clot obstructs the flow of blood to the brain, may be classified along with heart disease as a stress-related ailment of the circulatory system. Chronic obstructive lung diseases include emphysema, cystic fibrosis, and other conditions with widely differing causes that nonetheless all affect the same organ in the same way.
The remainder of the list includes two non-disease-related conditions (accidents at no. 5 and suicide at no. 8), three more noninfectious diseases (diabetes mellitus at no. 7, kidney disease at no. 9, and chronic liver disease and cirrhosis at no. 10); and just one infectious-disease-related set of causes. The latter, no. 6, consists of two diseases, pneumonia and influenza, which may be the result of another type of infection, the only infectious condition that poses a serious threat in the Western world today: AIDS, or acquired immunodeficiency syndrome.
The essay Disease classifies noninfectious diseases as follows: hereditary or genetic diseases (e.g., hemophilia, discussed later in this essay); glandular diseases, or conditions involving a group of cells that filters material from the blood (e.g., diabetes mellitus, also included in the present essay); dietary diseases (see the essays Vitamins and Nutrients and Nutrition); cancers (discussed here); stress-related diseases (see Disease); autoimmune diseases (see Immunity and Immunology); and diseases of unknown origin (see Immunity and Immunology).
This is one way of dividing up ailments, and it happens to be the method applied in the essays of this book that deal with diseases. This method has the advantage of illustrating the wide variation in noninfectious diseases, but it would not necessarily be the best model to use for an in-depth professional study of disease. Scientists who study illnesses typically use one of several methods of classification that, while less broadly based than the one used here—and perhaps less interesting as well—are more efficient, because they group all diseases according to the same characteristics.
Among the systems doctors and scientists use for grouping diseases are topographic; anatomic; physiological; pathologic; etiologic, or causal; and epidemiological classifications. Topographic classification refers to bodily region or system: for instance, the circulatory system, the neurological system, and so on. The second method, anatomic (by organ or tissue), also uses parts of the body as a criterion for classification. The designation of heart and lung diseases, used earlier in discussing leading causes of mortality in the United States, is an example of this system.
Physiological classifications divide diseases in terms of function or effect (for example, metabolic disorders, some of which are discussed in Metabolism), while pathologic classifications separate diseases by the nature of the process that the disease takes: for example, inflammatory diseases. Etiologic, or causal, classifications are used most commonly in discussing infectious diseases, where broad types of causes can include viruses, bacteria, or other types of parasites. Likewise, epidemiological classifications usually refer to infectious diseases. Epidemiology is an area of the medical sciences devoted to the study of disease, including its incidence, distribution, and control within a population.
Although we are accustomed to hearing of "cancer" as though it were one disease, it is actually many diseases, close to 100 in number. Some of the most common varieties include skin, lung, and colon cancer, as well as breast cancer in women and prostate cancer in men. Blood and lymph node cancers, known as leukemias and lymphomas, respectively, are widespread, whereas cancer of the kidneys, ovaries, uterus, pancreas, bladder, and rectum are included among the cancers that most often affect Americans.
As this listing suggests, most cancers attack either body parts or systems and therefore are often classified anatomically or topographically; yet several characteristics unite these conditions. Cancer strikes the genes, which are carriers of genetic information that make up part of DNA (deoxyribonucleic acid), a molecule that appears in all cells. By gaining control at this level, the cancer is like a terrorist who has established a grip on all the communication or transportation systems in a country.
Many genes produce proteins that play a part in controlling the processes of cell growth and division. An alteration, or mutation, to the DNA molecule can disrupt the genes and produce faulty proteins, causing the cells to become abnormal and multiply. The abnormal cell begins to divide uncontrollably and eventually forms a new growth, known as a tumor, or neoplasm. In a healthy person, the immune system can recognize the neoplastic cells and destroy them before they have a chance to divide. Some mutant cells may escape immune detection, however, and survive to become tumors or cancers. (The immune system is discussed in Immunity and Immunology.)
Tumors can be either benign or malignant. A benign tumor is slow growing, does not spread or invade surrounding tissue, and, once removed, usually does not recur. A malignant tumor, on the other hand, invades surrounding tissue and spreads to other parts of the body. Therefore, even if the malignant tumor is removed, if the cancer cells have spread to the surrounding tissues, cancer will return. If the cancer cells are allowed to keep growing in number, migrating from the site of origin and spreading throughout the body, they eventually will kill the patient.
A majority of cancers are caused by changes in the cell's DNA because of damage from the environment. Environmental factors that are responsible for the initial mutation in DNA are called carcinogens, and there are many types, which we discuss shortly. Additionally, some cancers have a genetic basis: in other words, a person can inherit faulty DNA from his or her parents, which can predispose the patient to cancer of one kind or another. While there is scientific evidence that both factors (environmental and genetic) play a role, less than 10% of all cancers are purely hereditary.
There are several different types of cancers. In addition to leukemias and lymphomas, mentioned earlier, there are carcinomas, or cancers that arise in the epithelium (the layers of cells covering the body's surface and lining the internal organs and various glands). These types alone account for about 90% of all cancers. Some forms of skin cancer are melanomas, which typically originate in the pigment cells. Other forms of cancer include sarcomas (cancers of the supporting tissues of the body, e.g., bone, muscle, and blood vessels), and gliomas, or cancers of the nerve tissue.
Many signs indicate the onset of cancer, among them, changes in the size, color, or shape of a wart or a mole; a sore that does not heal; or persistent cough, hoarseness, or sore throat. Many other diseases can produce similar symptoms, however, and for this reason it is important for a person to visit a doctor for regular checkups and diagnosis. Usually, diagnosis calls for fairly routine physical examination, though in the case of cancers of the reproductive organs, "routine" can still be plenty invasive.
Doctors examining women for cancers of the ovaries, uterus, cervix, and vagina must palpate the internal organs—that is, examine them by touch. For males, inspection of the rectum and the prostate is included in the physical examination. The doctor inserts a gloved finger into the rectum and rotates it slowly to feel for any growths, tumors, or other abnormalities. The doctor also palpates the testicles to identify any lumps, thickening, or differences in the size, weight, or firmness. Such examinations, as well as diagnoses for certain other types of cancer in private parts (namely, colon cancer), can be less than pleasant, but they are certainly preferable to an early and painful death.
If the patient has an abnormality that could be indicative of cancer, the doctor may order diagnostic tests. These tests may include laboratory studies of sputum or saliva, blood, urine, and stool (feces). To locate tumors, such imaging tests as computerized tomography (CT) scans, magnetic resonance imaging (MRI), ultrasound, or fiber-optic scope examinations may be used. The most definitive diagnostic test, however, is the biopsy, in which a piece of tissue is surgically removed for examination under a microscope. Besides confirming whether a patient has cancer, the biopsy also provides information about the type of cancer, the stage it has reached, the aggressiveness of the cancer, and the extent of its spread.
Screening examinations, conducted regularly by health care professionals, can result in the detection of cancers at an early stage. In addition, advances in molecular biology (an area of biology concerned with the physical and chemical basis of living matter) and cancer genetics have led to the development of several tests for assessing one's risk of developing cancers. These new techniques include genetic testing, in which molecular probes are used to identify mutations in certain genes that have been linked to particular cancers. At present, however, there are limitations to genetic testing, a fact that emphasizes the need for better strategies of early detection.
Although there is as yet no cure for cancer, there are treatments designed to remove as much of the tumor or tumors as possible and to prevent the recurrence or spread of the cancer. Cancer treatment can take many different forms, including surgery, radiation, chemotherapy, immunotherapy, hormone therapy, and bone-marrow transplantation. Physicians recommend specific treatments based on the needs, condition, and illness of the particular patient.
Surgery, the most frequently used form of cancer treatment, involves the removal of the visible tumor. It is most effective when a cancer is small and confined to one area of the body. Radiation, which kills tumor cells by bombarding them with high-energy waves or particles, may be used alone in cases where a tumor is unsuitable for surgery. More often, however, it is used in conjunction with surgery and chemotherapy or with drugs to kill cancer cells. While it can be highly effective, chemotherapy may be physically difficult for the patient and may have side effects, including temporary hair loss.
Immunotherapy uses the body's own immune system to destroy cancer cells. The various immunological agents being tested include substances produced by the body as well as vaccines. Unlike traditional vaccines, cancer vaccines do not prevent cancer; instead, they are designed to train the immune system of the cancer patient's body to attack and destroy cancer cells. Hormone therapy is standard treatment for some types of cancers that are hormone-dependent and which grow faster in the presence of particular hormones. Among them are cancers of the prostate, breast, and uterus, and the therapy is designed to block the production or action of the hormones involved.
A particularly aggressive form of treatment is bone-marrow transplantation, which involves taking tissue from within a donor's bone cavities, where blood-forming cells are located, and transplanting it into the patient. In addition, cancer patients may use massage, reflexology, herbal remedies, and other forms of treatment dubbed "alternative," meaning that they usually are not recognized by the mainstream of the medical profession. (Lack of official recognition does not necessarily mean anything: many people have benefited from alternative cancer treatments.)
One of every four deaths in the United States is from cancer, and each year more than a million Americans are diagnosed with some form of the disease. Of these people, about half will die of the disease. Cancer can attack anyone, even children, though, fortunately, cases of cancer in very young patients are the exception. Most cases are seen in middle-aged or older adults. Although scientists are a long way from being able to predict who will get cancer (much less effectively prevent it), they have identified numerous risk factors. A risk factor is not necessarily a cause per se (though it may be); rather, if there seems to be a link between a particular behavior and a specific disease, that behavior is referred to as a risk factor for that disease. Major risk factors for cancer are tobacco use, alcohol consumption, diet, certain types of sexual and reproductive behavior, infectious agents, family history, occupation, environment, and pollution.
Approximately two-fifths of all cancer deaths annually are linked to tobacco use and excessive alcohol consumption. In addition to the relationship between smoking and lung cancer (80-90% of all lung cancer patients are smokers), smoking also has been shown to be a contributory factor to a whole host of other cancers. Moreover, scientists have established that second-hand smoke (or passive smoking) can increase one's risk of cancer. Nor is "smokeless tobacco" a safe alternative to cigarettes: snuff and chewing tobacco have been associated with countless cases of mouth cancer.
Excessive drinking is a risk factor in liver cancer and other illnesses, and the deadly combination of tobacco and excessive alcohol use significantly increases the chances that a person will contract mouth, pharynx, larynx, or esophageal cancer. Tobacco and alcohol are not the only cancer-related agents that people take into their bodies: about one-third of all cancer deaths annually are related to the things people eat. For example, immoderate intake of fat, leading to obesity, has been associated with cancers of the breast, colon, rectum, pancreas, prostate, gallbladder, ovaries, and uterus.
There are even varieties of cancer that are linked to contagious diseases. Since the mid-1970s, scientists have obtained evidence showing that approximately 15% of all cancer deaths worldwide can be traced to viruses, bacteria, or parasites. One such pathogen, the human papilloma virus, is sexually transmitted. Having too many sex partners and becoming sexually active too early have been shown to increase one's chances of contracting cancer of the cervix. (On the other hand, women who do not have children or those who have them late in life have a higher risk of both ovarian and breast cancer.)
Certain cancers, including those of the breast, colon, ovaries, and uterus, recur generation after generation in some families, and therefore family history and genetics cannot be ruled out as risk factors. In addition, less well-known cancers, such as the eye condition known as retinoblastoma, have been traced to certain genes that can be tracked within a family. Thus, it is possible that inheriting particular genes makes a person susceptible to certain cancers.
Cancers arising from occupational hazards make up a particularly significant group. They account for 4% of all cancer deaths, and there is a certain poignancy in the fact that these cancer victims contracted their illness not by drinking or smoking or bad eating but simply by earning a living. Nowhere was this poignancy more evident than in some of the first documented cases of cancer arising from occupational causes.
In 1775 the English surgeon Percivall Pott (1714-1788) described the high incidence of cancer of the scrotum among former chimney sweeps, most of them men in their twenties. This was bad enough, but the fact that they had contracted the cancer much earlier hinted at a reality even more grim. In the harsh early days of the Industrial Revolution, before child labor laws had even been imagined, boys as young as four—orphans or children of desperately poor families—were put to work cleaning the insides of chimneys. The chimneys were so narrow that only children could clean them, and because of the tight fit, even small boys were unable to wear any clothes while doing their job. As a result, soot became embedded in their skin, and, since few people bathed, it stayed there, a silent killer whose effects became apparent only many years later.
In addition to helping make English society aware of the injustices put upon members of its lowest classes, Pott's research introduced the medical world to the concept of occupational health. Still, until the last third of the twentieth century, large numbers of industrial workers in the West labored at occupations with built-in cancer hazards. Thus, asbestos workers contracted lung cancer at a high rate, and a link became apparent between bladder cancer and dye and rubber industries. Likewise, connections were established between skin or lung cancer and the jobs of smelters, gold miners, and arsenic workers; between leukemia and glue and varnish workers' occupations; between liver cancer and the business of PVC (polyvinyl chloride) manufacturers; and between lung, bone, and bone marrow cancer and the work of radiologists and uranium miners.
Radiation itself is an environmental hazard that affects not only workers in those specialized industries just named but also anyone who has been exposed to radioactive materials. Fortunately, this is less of a hazard today, thanks to numerous bans on nuclear testing; nonetheless, radiation—including ultraviolet radiation from
In contrast to the boys Pott treated, sufferers from hemophilia in the past often came from the highest echelons of society. A hereditary disease that primarily affects males, hemophilia was passed down through many royal lines, with the females acting as carriers and some of the males in the bloodline becoming victims of the disease. England's Queen Victoria (1819-1901) had several sons with hemophilia who died before they had the opportunity to become king. Her nephew, Russia's Czar Nicholas II, had a son who was hemophiliac as well. The boy's affliction caused his mother to seek the help of the charismatic "healer" Grigori Rasputin, whose close involvement with the royal family fed Russian discontent and helped contribute to the overthrow of the czar in 1917. (Ironically, the boy died not from hemophilia but perished, along with his family, before a Bolshevik firing squad.) Thus, once again (as noted in Disease), disease affected the course of history.
Its name taken from Greek and Latin words that together mean "love of bleeding," hemophilia
Much more prevalent than the genetic disorder hemophilia, or any one form of cancer, is diabetes mellitus, a serious condition caused by an absence of, or an insufficient amount of, insulin. Insulin, a hormone produced by the pancreas in varying amounts, helps maintain a normal concentration of glucose, or blood sugar (see Carbohydrates). Diabetes in general is a glandular disease, the gland in this case being the pancreas, but type 1 diabetes also is considered an autoimmune disorder because the body's own immune system destroys its insulin-producing cells. (See Immunity and Immunology and The Immune System for more about autoimmune disorders.)
Diabetes prevents the body from putting glucose to use, and instead large amounts of it are excreted in the urine. The word diabetes means "siphon," a reference to one of its major symptoms: frequent urination in an attempt to expel glucose. The urine itself is full of sugar; hence the term mellitus, meaning "honey." (In this vein, it is worth mentioning Chen Chuan, an eighth-century Chinese physician who surely qualifies as one of history's most dedicated scientists. He was the first to describe the sweetness of urine in patients suffering from diabetes, presumably as the result of firsthand research that went above and beyond the call of duty.)
More than 12 million Americans, and some 100 million people worldwide, are affected by diabetes. That number is increasing by 5-6% annually, primarily the result of the population's increased longevity, combined with other factors such as increasing obesity and consumption of rich, processed, and carbohydrate-loaded foods (and drinks such as beer). Approximately 300,000 deaths each year are attributed to diabetes, which is of two principal varieties. Type 1, or insulin-dependent diabetes, is present at birth, is characterized by insulin deficiency, and normally is treated by taking insulin injections. Type 2, or non-insulin-dependent diabetes, arises not at birth, but somewhat later (though it can occur in childhood) and then among people who have normal insulin levels. (With type 2 diabetes, the problem is the body's inability to use its insulin efficiently.) Type 2, which typically stems from dietary causes—is preventable, but it is not treatable by insulin injections. Type 2 diabetes also may temporarily affect pregnant women, who may experience heightened glucose levels in a condition known as gestational diabetes. (There is another kind of diabetes altogether, a rare condition known as diabetes insipidus, which involves inadequate production of another hormone, vasopressin.)
One might wonder why problems with blood sugar could be so serious as to kill a million people every three years and to qualify diabetes as one of the leading killers in America. The reason is that the body depends on glucose as a source of immediate energy, and in the absence of usable glucose, it begins instead to use its fat cells. This rapid burning of fat produces a surplus of organic compounds known as ketones, and ketone accumulation brings about an accumulation of acids in the blood, a condition known as ketoacidosis. Severe ketoacidosis can cause nausea, vomiting, and a loss of consciousness, or diabetic coma. If the patient does not receive a shot of insulin, he or she can die. Thus, the symptoms displayed by Julia Roberts's diabetic character in her first major movie, Steel Magnolias (1989), were not overdone. Diabetes can also bring about other conditions, including blindness, kidney diseases, and long-term organ damage.
As difficult as life is for diabetics today, it is infinitely better than it was before 1921. That was the year when the Canadians Frederick Banting (1891-1941), a surgeon, and Charles Herbert Best (1899-1978), a physiologist, isolated insulin. Thanks to their work, and the subsequent development of insulin therapy—typically using insulin harvested from cows or pigs—deaths from ketoacidosis and diabetic coma declined. A person with type 1 diabetes, formerly consigned to a dramatically shortened life of misery, could hope to have something approaching a normal existence. Today diabetes remains a life-shortening illness, and doctors and scientists continue to search for a cure, but ever since 1921, the lot of those with diabetes has been improving steadily. (Banting was later knighted and shared the 1923 Nobel Prize in physiology and medicine for his achievement. Best, however, in one of history's great snubs, received neither honor, because he had not earned his doctorate at the time of the discovery.)
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A substance or agent that induces the development of cancer.
A general term for any condition that impairs the normal functioning of an organism.
Deoxyribonucleic acid, a molecule in all cells, and many viruses, containing genetic codes for inheritance.
A term for a disease that is communicable or contagious and comes from outside the body. Compare with intrinsic.
A unit of information about a particular heritable (capable of being inherited) trait that is passed from parent to offspring and stored in DNA molecules called chromosomes.
The study of hereditary traits passed down from one generation to the next through the genes.
A cell or group of cells that filters material from the blood, processes that material, and secretes it either for use again in the body or to be eliminated as waste.
A term for a disease that is not communicable or contagious and comes from inside the body. Compare with extrinsic.