When a person's body is not working normally, that person may have a disease. Diseases, also called illnesses or sicknesses, are conditions that make people unhealthy. Doctors use patients' symptoms (such as pain) and signs (such as blood pressure) to identify diseases. Human diseases fall into two major groups: infectious diseases and noninfectious diseases. Infectious diseasesInfectious diseases are contagious, or catching. These diseases are transmitted, or spread, in a number of ways. Some are transmitted by direct contact between humans. Other infectious diseases, including malaria and yellow fever, are transmitted by animals, such as mosquitoes. Still other infectious diseases are transmitted through the air or through contaminated water and contaminated food. CausesInfectious diseases are caused by pathogens. Pathogens are microscopic substances such as viruses, bacteria, fungi, and protozoans. To cause a disease a pathogen must first enter a person's body. Pathogens can enter through any opening in the body—the nose or the mouth, for example—or through a break in the skin. Once a pathogen has entered the body, the person is said to be infected. Inside a person's body, pathogens grow and multiply. That growth can cause harmful changes to the whole body or to one or more of its parts. ExamplesOne of the most common diseases, the common cold, is caused by a virus. Some other diseases caused by viruses are influenza (the flu), chicken pox, measles, and AIDS. A common disease caused by bacteria is strep throat. Other diseases caused by bacteria include tuberculosis, Lyme disease, and food poisoning. Fungi cause such infectious skin diseases as athlete's foot and ringworm. Fungi can also cause serious diseases inside a person's body. Protozoans cause such infectious diseases as malaria, amebic dysentery, and sleeping sickness. The protozoan that causes malaria infects the blood. The one that causes amebic dysentery infects the intestines. The one that causes sleeping sickness infects the brain. Some infectious diseases are caused by larger organisms. For example, a flatworm called a blood fluke causes a serious blood disease in people. Another flatworm, called a tapeworm, infects the intestines. TreatmentDoctors can often cure diseases caused by bacteria with antibiotics. Various chemicals are used to treat diseases caused by viruses, fungi, and protozoans. Doctors also use vaccines to prevent many infectious diseases, especially those caused by bacteria and viruses. Noninfectious diseasesMany human diseases are not caused by living things that invade the body. These diseases are called noninfectious. They are not catching, which means they cannot be spread through direct contact, animals, or contaminated air or food. Some people have noninfectious diseases from the time of birth. Others develop noninfectious diseases later in life. CausesSome noninfectious diseases are inherited, or handed down from parents to children. A person's lifestyle can cause certain noninfectious diseases. Environmental dangers or pollution can also cause noninfectious diseases. Some noninfectious diseases, such as cancer and diabetes, have several possible causes. Sometimes doctors do not know the exact cause of a noninfectious disease. ExamplesInherited noninfectious diseases are passed down through families. Parents might not have the disease. They might just carry the gene that causes the disease in their children. Sickle cell anemia, which affects the shape of blood cells, is a common inherited disease. Hemophilia is another inherited disease. The blood of people with hemophilia clots more slowly than does the blood of other people. Noninfectious diseases that can be caused by lifestyle or the environment include some types of cancer and some types of heart disease. People who smoke cigarettes are more likely to get lung cancer than nonsmokers. Those who live close to polluting factories or toxic waste dumps might also get cancer or other diseases. People who eat too many fatty foods and do not exercise enough are in danger of developing heart disease. Diabetes mellitus is an example of a noninfectious disease that has several possible causes. In people with this form of diabetes, the organ called the pancreas does not make enough of the chemical insulin, or the body cannot use the insulin that it produces. (Insulin keeps the amount of sugar in the blood from rising to unsafe levels.) Diabetes mellitus may be inherited or caused by obesity. It can also be caused by pregnancy or other diseases. Alzheimer's disease is a noninfectious disease whose exact causes are unknown. People with Alzheimer's disease slowly lose their memories. Their mental functions fail as nerve cells in the brain die. Scientists think that defects in certain genes may lead to the disease. TreatmentScientists are experimenting with ways to cure inherited diseases and noninfectious diseases with unknown causes. In the meantime, some of those diseases can be treated. For example, doctors treat sickle cell anemia with a drug that helps to create normal blood cells. They treat hemophilia with medicines that speed up the clotting of blood. People with diabetes mellitus get insulin shots and reduce the amount of sugar they eat. Cancer treatments include radiation therapy and chemotherapy. Surgeons can treat heart disease by opening up blocked blood vessels or by replacing them. People with Alzheimer's disease can take drugs that help to stabilize their symptoms. |