Diseases are any harmful change that interferes with the normal appearance, structure, or function of the body or any of its parts. Since time immemorial, disease has played a role in the history of societies. It has affected and has been affected by economic conditions, wars, and natural disasters. An epidemic of influenza that swept the globe in 1918 killed between 20 million and 40 million people. Within a few months, more than 500,000 Americans died^more than were killed during World War I (1914-1918), World War II (1939-1945), the Korean War (1950-1953), and the Vietnam War (1959-1975) combined. Diseases have diverse causes, which can be classified into two broad groups: communicable and noncommunicable. Communicable diseases can spread from one person to another and are caused by microscopic organisms that invade the body. Noncommunicable 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, and come on suddenly and last for no more than a few weeks. Other diseases, such as arthritis, are chronic, consistent for months or years, or reoccur frequently. Every disease has certain characteristic effects on the body. Some of these effects, include fever, inflammation, pain, fatigue, dizziness, nausea, and rashes, are evident to the patient. These symptoms offer important clues that help doctors and other health care professionals make a diagnosis. Many times, the symptoms point to several possible disorders. In those cases, doctors rely on medical tests, such as blood examinations and X rays, to confirm the diagnosis. Communicable diseases are caused by microscopic organisms. 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. Also, it has been theorized that some proteins called prions may cause infectious diseases. Bacteria are microscopic single-celled organisms at least 1 micron long. Some bacteria species are harmless to humans, many are beneficial. But some are pathogens, including those that cause cholera, diphtheria, leprosy, plague, pneumonia, strep throat, tetanus, tuberculosis, and typhoid fever. The bacteria that are harmless and live in or on you are called resident bateria. Viruses are tens or hundreds of times smaller than bacteria. They are not cellular, but consist of a core of genetic material surrounded by a protective coat of protein. Viruses are able to survive and reproduce only in the living cells of a host. Once a virus invades a living cell, it directs the cell to make new virus particles. These new viruses are released into the surrounding tissues, and seek out new cells to infect. The roll call of human diseases caused by viruses includes mumps, measles, influenza, rabies, hepatitis, poliomyelitis, smallpox, AIDS, and certain types of cancer. Fungi are a varied group of generally small organisms that get their food from living or dead organic matter. They germinate from reproductive cells called spores, which often have a thick, resistant outer coat that protects against unfavorable environmental conditions. This enables spores to survive for long periods of time, which adds to the difficulty of treating fungal infections. 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. Protozoans are single-celled, animal-like organisms that live in moist environments. The most infamous pathogenic protozoans are species of the genus Plasmodium, which cause malaria, an infectious disease responsible for over 2 million deaths worldwide each year. Members of the genus Trypanosoma produce trypanosomiasis, also known as African sleeping sickness, and Chagas' disease. Other protozoans cause giardiasis, leishmaniasis, and toxoplasmosis. Some pathogens are spread from one person to another by direct contact. They leave the first person through body openings, mucous membranes, and skin wounds, and they enter the second person through similar channels. The viruses that cause respiratory diseases such as influenza and the common cold are spread in moisture droplets when an infected person coughs or sneezes. A hand that was used to cover the mouth while coughing contains viruses that may be passed to doorknobs, so that the next person to touch the doorknob has a chance of picking up the infectious agent. The bacteria that cause some sexually transmitted diseases, including gonorrhea and syphilis, are transmitted during sexual contact. Other pathogens involve an intermediary carrier, such as an insect. The malarial parasite, for example, spends part of its life cycle in mosquitoes, then enters a person's bloodstream when the mosquito bites the person. Many pathogens are spread through contaminated food and water. Other pathogens can be passed on by contaminated food or water. Noncommunicable diseases not known to be caused by infectious agents include the three leading killers in
and other developed countries: heart disease, most cancers, and cerebrovascular disease. Noncommunicable 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's disease, and Alzheimer's disease, involve the progressive breakdown of tissues and loss of function of parts of the body. Joints gradually become stiff; bones become brittle; blood vessels become blocked by deposits of fat. The incidence of these problems increases with age, and, in at least some cases, progression can be slowed by good health habits. There are many ways to prevent these diseases. The skin and mucous membranes form the body's first line of defense against disease. Most microscopic pathogens, or microbes, cannot pass through unbroken skin, although they can easily enter through cuts and other wounds. Mucous membranes protect internal organs that are connected with the outside of the body. These membranes, which line the respiratory, digestive, urinary, and reproductive tracts, secrete mucus, which traps microbes. The mucus may then be expelled from the body, perhaps in a cough or sneeze or in feces. If the mucus is swallowed, digestive juices kill the microbes. Small hairlike projections on the lining of the nose, throat, and bronchial tubes work in conjunction with mucus to trap and remove foreign substances. In the ears, tiny hairs plus a sticky wax defend against the entry of germs. Tears secreted by the lachrymal gland wash away germs and other small objects that may enter the lid area of the eye. Tears also contain a protein that kills certain germs. If a pathogen breaks through the body^s outer barriers, the defenses of the immune system spring into action. Some of these defenses are effective against a variety of invaders, while others are formed to fight a specific organism. White blood cells called phagocytes constantly travel through the bloodstream on the lookout for foreign objects. If they come upon a microorganism, they surround, engulf, and digest it. During the 20th century, the importance of vitamins and other nutrients in preventing disease was recognized. Antibiotics, sulfa drugs, blood types, and genes that cause disease were discovered. A host of diagnostic and surgical tools were created that incorporated inventions such as X rays, fiber optics, lasers, and computers. Techniques such as organ transplantation, kidney dialysis, dental implants, gene therapy, and fetal surgeries were introduced. Thousands of new drugs were developed to treat everything from ulcers to zinc malabsorption. At the beginning of the 20th century, people in the had an average life span of about 50 years. By the time the century neared its close, average life span had risen to 76 years. Other developed countries experienced similar increases. Much of the credit for these longer life spans, and for the good health that accompanies them, is due to the conquering of diseases, thanks to vaccines, antibiotics, sophisticated surgical tools, and other medical miracles. The challenges ahead include bringing the benefits of this medical knowledge to all peoples of the world, and expanding on current knowledge in order to understand, treat, and prevent the diseases that still confront us.