Disease

Human Disease, in medicine, is any harmful change that interferes with the normal appearance, structure, or function of the body or any of its parts. Diseases have diverse causes, which can be classified into two broad groups: infectious and noninfectious. Infectious diseases can spread from one person to another and are caused by microscopic organisms that invade the body. Noninfectious diseases are not communicated from person to person and do not have, or are not known to involve, infectious agents. Some diseases, such as the common cold, are acute, coming on suddenly and lasting for no more than a few weeks. Other diseases, such as arthritis, are chronic, persisting for months or years, or recurring frequently. Infectious diseases are caused by microscopic organisms commonly called germs. Physicians refer to these disease-causing organisms as pathogens. Pathogens that infect humans include a wide variety of bacteria, viruses, fungi, protozoans, and parasitic worms. In addition, it has been theorized that some proteins called prions may cause infectious diseases. These include cholera, diphtheria, leprosy, plague, pneumonia, strep throat, tetanus, tuberculosis, and typhoid fever. The roll call of human diseases caused by viruses includes mumps, measles, influenza, rabies, hepatitis, poliomyelitis, smallpox, AIDS, and certain types of cancer. Some fungi are external parasites of humans, causing skin conditions such as ringworm, athlete's foot, and jock itch. Other fungi invade internal tissues; examples include yeast that infect the genital tract and several fungi species that cause a type of pneumonia. Noninfectious diseases include heart disease, most cancers, and cerebrovascular disease (decreased blood circulation in the brain). Noninfectious illnesses include disorders as terrifying as Alzheimer's disease, which robs victims of their memory and their ability to reason, and as pesky as poison ivy. Degenerative disorders, including arthritis, Parkinson disease, and Alzheimer's disease,involve the progressive breakdown of tissues and loss of function of parts of the body. See also: Hereditary and Congenital Diseases; Immunological Diseases, Deficiency Diseases. related articles:

• mental illness
• physical fitness
• viruses that cause disease
• bacteria that cause disease
• cancer
• heart disease
• disease terminology
• contagious and infectious deasease
• incurable diseases
• treatment

Ribonucleic Acid

Ribonucleic Acid (RNA), genetic material of certain viruses (RNA viruses) and, in cellular organisms, the molecule that directs the middle steps of protein production. In RNA viruses, the RNA directs two processes—protein synthesis (production of the virus's protein coat) and replication (the process by which RNA copies itself). In cellular organisms, another type of genetic material, called deoxyribonucleic acid (DNA), carries the information that determines protein structure. But DNA cannot act alone and relies upon RNA to transfer this crucial information during protein synthesis (production of the proteins needed by the cell for its activities and development).

Like DNA, RNA consists of a chain of chemical compounds called nucleotides. Each nucleotide is made up of a sugar molecule called ribose, a phosphate group, and one of four different nitrogen-containing compounds called bases. The four bases are adenine, guanine, uracil, and cytosine. These components are joined together in the same manner as in a deoxyribonucleic acid (DNA) molecule. RNA differs chemically from DNA in two ways: The RNA sugar molecule contains an oxygen atom not found in DNA, and RNA contains the base uracil in the place of the base thymine in DNA.

Deoxyribonucleic Acid

Deoxyribonucleic Acid (DNA), genetic material of all cellular organisms and most viruses. DNA carries the information needed to direct protein synthesis and replication. Protein synthesis is the production of the proteins needed by the cell or virus for its activities and development. Replication is the process by which DNA copies itself for each descendant cell or virus, passing on the information needed for protein synthesis. In most cellular organisms, DNA is organized on chromosomes located in the nucleus of the cell.

Cancer

Cancer (medicine), any of more than 100 diseases characterized by excessive, uncontrolled growth of abnormal cells, which invade and destroy other tissues. Cancer develops in almost any organ or tissue of the body, but certain types of cancer are more lethal than others.

HOW CANCER DEVELOPS

A healthy human body is composed of 30 trillion cells, most of which are in constant turnover as cells die and others reproduce to replace them in an orderly fashion. Healthy cells of the skin, hair, lining of the stomach, and blood, for example, regularly reproduce by dividing to form two daughter cells (see Mitosis). This cell division cycle proceeds under the regulation of the body’s intricately tuned control system. Among other functions, this control system ensures that cells only divide when needed, so that organs and tissues maintain their correct shape and size. Should this system fail, a variety of backup safety mechanisms prevent the cell from dividing uncontrollably. In order for a cell to become cancerous, every one of these safety mechanisms must fail.

Cancer begins in genes, bits of biochemical instructions composed of individual segments of the long, coiled molecule deoxyribonucleic acid (DNA). Genes contain the instructions to make proteins, molecular laborers that serve as building blocks of cells, control chemical reactions, or transport materials to and from cells. The proteins produced in a human cell determine the function of each cell, and ultimately, the function of the entire body.

In a cancerous cell, permanent gene alterations, or mutations, cause the cell to malfunction. For a cell to become cancerous, usually three to seven different mutations must occur in a single cell. These genetic mutations may take many years to accumulate, but the convergence of mutations enables the cell to become cancerous.

Globular proteins

Unlike fibrous proteins, globular proteins are spherical and highly soluble. They play a dynamic role in body metabolism. Examples are albumin, globulin, casein, hemoglobin, all of the enzymes, and protein hormones. The albumins and globulins are classes of soluble proteins abundant in animal cells, blood serum, milk, and eggs. Hemoglobin is a respiratory protein that carries oxygen throughout the body and is responsible for the bright red color of red blood cells. More than 100 different human hemoglobins have been discovered, among which is hemoglobin S, the cause of sickle-cell anemia, a hereditary disease suffered mainly by blacks.

A Enzymes

All of the enzymes are globular proteins that combine rapidly with other substances, called substrate, to catalyze the numerous chemical reactions in the body. Chiefly responsible for metabolism and its regulation, these molecules have catalytic sites on which substrate fits in a lock-and-key manner to trigger and control metabolism throughout the body.

B Protein Hormones

These proteins, which come from the endocrine glands, do not act as enzymes. Instead they stimulate target organs that in turn initiate and control important activities—for example, the rate of metabolism and the production of digestive enzymes and milk. Insulin, secreted by the islands of Langerhans, regulates carbohydrate metabolism by controlling blood glucose levels. Thyroglobulin, from the thyroid gland, regulates overall metabolism; calcitonin, also from the thyroid, lowers blood calcium levels. Angiogenin, a protein structurally determined in the mid-1980s, directly induces the growth of blood vessels in tissues.

C Antibodies

Also called immunoglobulins, antibodies make up the thousands of different proteins that are generated in the blood serum in reaction to antigens (body-invading substances or organisms). A single antigen may elicit the production of many antibodies, which combine with different sites on the antigen molecule, neutralize it, and cause it to precipitate from the blood.

D Microtubules

Globular proteins can also assemble into minute, hollow tubes that serve both to structure cells and to conduct substances from one part of a cell to another. Each of these microtubules, as they are called, is made up of two types of nearly spherical protein molecules that pair and join onto the growing end of the microtubule, adding on length as required. Microtubules also make up the inner structure of cilia, the hairlike appendages by which some microorganisms propel themselves.