A disease is an abnormal condition of an organism that impairs the organism's function in whole or in part and is identified by characteristic signs or symptoms. In reference to human beings, "disease" is often used broadly to refer to any condition that causes discomfort, dysfunction, distress, social problems, and/or death to the person afflicted, or similar problems for those in contact with the person. In this broader sense, disease sometimes includes injuries, disabilities, disorders, syndromes, infections, isolated symptoms, deviant behaviors, and atypical variations of structure and function, while in other contexts and for other purposes each of these may be considered its own distinguishable category.
While many diseases are biological processes with observable alterations of organ function or structure, others primarily involve alterations of behavior associated with an abnormal condition of the mind.
Causes of diseases, both physical and mental are various, including infectious agents such as bacteria and viruses, environmental poisons, genetic defects, poor diet, unhealthy habits like smoking or overeating, physical or mental stress, auto-immune reactions, a combination of these factors, and, from an Eastern perspective, imbalance among the body's systems and organs.
Some diseases are preventable through the exercise of personal responsibility in the pursuit of health and wellness. Among factors important for prevention of such diseases are good nutrition, exercise, stress management, a healthy environment, wise use of medical resources, good human relationships, a positive outlook on life, and having a spiritual or religious dimension to ones life. Alternative and Eastern medical approaches—whether for humans, animals, or plants—place primary emphasis on models and methods of health maintenance while including diseases and their treatment as a subset of that larger medical model.
Pathology is the study of diseases, and nosology is the systematic classification of diseases. The body of knowledge about human diseases and their treatments is the territory of allopathic or Western medicine. The study of diseases affecting domestic animals, wildlife, livestock, and exotic animals is veterinary medicine. Plants as well can suffer from a variety of afflictions such as infection, nutrient deficiency, or deleterious mutation. Plant pathology is the study of diseases affecting plants.
For diseases with known causes, disease prevention is a matter of personal responsibility according to the safety precautions and lifestyle choices one makes. Individuals with a religious orientation tend to emphasize spiritual well-being as a factor in health promotion and disease prevention. Religions recognize various spiritual laws such as to love, to give, and to live for the sake of others, which are important to one's spiritual health. When one is self-centered, one violates such universal principles, leading the body to be more apt to be susceptible to disease. Following such spiritual principles is also fundamental to having good human relationships and a positive attitude in life.
Classifying a condition as a disease is a social act of valuation, and may change the social status of the person with the condition (the patient). Some conditions (known as culture-bound syndromes) are only recognized as diseases within a particular culture. Sometimes the categorization of a condition as a disease is controversial within the culture.
Medical usage sometimes distinguishes a disease, which has a known specific cause or causes (called its etiology), from a syndrome, which is a collection of signs or symptoms occurring together. The distinction between disease and syndrome, however, is ambiguous, as many conditions whose causes have been identified, continue to be referred to as syndromes, while numerous conditions of unknown etiology are referred to as diseases.
Illness, although often used to mean disease, can also refer to a person's perception of personal health, regardless of whether the person in fact may have a disease. A person without any disease may feel unhealthy and believe he has an illness. Another person may feel healthy and believe he does not have an illness even though he may have a disease such as dangerously high blood pressure, which may lead to a fatal heart attack or stroke.
The term symptom (from the Greek syn = con/plus and pipto = fall, together meaning co-exist) has two similar meanings in the context of physical and mental health:
A sign or symptom, then, is not necessarily defined by its nature, but rather by who observes it. The same feature may be noticed by both doctor and patient, and so is at once both a sign and a symptom. Some features, such as pain, can only be symptoms. A doctor can not feel a patient's pain (unless he is the patient). Others can only be signs, such as a blood cell count measured by a doctor in his/her laboratory.
Some symptoms (e.g. nausea) occur in a wide range of disease processes, whereas other symptoms are fairly specific for a narrow range of illnesses: for example, a sudden loss of sight in one eye has only a very limited number of possible causes. Some symptoms can be misleading to patients or the medical practitioner caring for them. For example, inflammation of the gallbladder quite often gives rise to pain in the right shoulder, which might (quite reasonably) lead a patient to attribute the pain to a non-abdominal cause such as muscle strain, rather than the real cause.
Some diseases, such as influenza, are contagious or infectious, and can be transmitted by any of a variety of mechanisms, including coughs and sneezes, sexual transmission, by bites of insects or other carriers of the disease, from contaminated water or food, and so forth.
The identification of a condition as a disease, rather than as simply a variation of human structure or function, can have significant social or economic implications. The controversial recognitions as diseases of post-traumatic stress disorder, also known as "shell shock"; repetitive motion injury or repetitive stress injury (RSI); and Gulf War syndrome have had a number of positive and negative effects on the financial and other responsibilities of governments, corporations, and institutions towards individuals, as well as on the individuals themselves.
A condition may be considered to be a disease in some cultures or eras but not in others. Oppositional-defiant disorder, attention-deficit hyperactivity disorder (ADHD), and, increasingly, obesity are conditions considered to be diseases in the United States and Canada today, but were not so considered decades ago and are not so considered in some other countries. Lepers were a group of afflicted individuals who were historically shunned and the term "leper" still evokes social stigma. Fear of disease can still be a widespread social phenomenon, though not all diseases evoke extreme social stigma.
The concept of disease prevention must begin with a clarification of what meaning of disease is being considered, since syndromes, conditions, or disorders with no known cause should reasonably be viewed differently than infectious diseases such as malaria or lifestyle diseases such as obesity. For diseases with a known and individually controllable cause, disease prevention is tied to personal responsibility. Among the factors that have been identified in the health promotion and wellness movements are good nutrition, exercise, stress management, wise use of medical resources, healthy environments, good human relationships, and a positive outlook on life.
A Harvard University publication on nutrition states that healthy eating can prevent 25 percent of all cancers and combined with exercise and being a nonsmoker can prevent up to 90 percent of adult onset diabetes.
Exercise has been shown to reduce risks for diseases such as heart disease, high blood pressure, diabetes, obesity, and colon cancer. In addition, exercise has been shown also to reduce risks for some mental diseases, such as depression.
Good stress management can also reduce the risk of and/or prevent disease. Stress can be a contributing or even primary factor to a number of pathologies or combination of pathologies including cardiovascular disease, mental diseases, internal organ diseases, or musculoskeletal diseases.
Wise use of medical resources includes having appropriate age related checkups to detect or catch diseases early on so they can be treated effectively. Some of the common preventive checkups include: screenings for blood pressure, cholesterol/HDL ratios, breast exams for women, and so forth. Taking vaccines is another medical resource, such as taking a vaccine for malaria before traveling to a region where malarial disease is prevalent.
Living in a healthy environment is becoming increasingly important to preventing disease. Access to clean water and sanitation, freedom from toxic chemical exposure, clean air, and so forth can help reduce and prevent disease.
Having satisfying human relationships and a positive outlook on life have been found to be a factor in the reduction and/or prevention of disease. In one study of 40,000 people performed by the U.S. Department of Health and Human Services, it was found that satisfying human relationships and job satisfaction were better predictors (inverse correlation) of heart disease than all other factors.
In general, self-responsibility for one’s health is a critical way to prevent disease. The person who takes responsibility for not smoking reduces risks for several diseases such as heart disease, stroke, lung cancer, and throat cancer, to name a few. In the same, way responsible alcohol consumption can reduce the risks for cirrhosis of the liver, alcoholism, and some mental diseases. In the same manner, taking responsibility to avoid addicting drugs can prevent disease. A person taking measures for responsible sexual behavior can also help prevent or reduce risks for several diseases, including HIV-AIDS. Using a mosquito proof net and using insect repellent while traveling in an region with insect-borne diseases can reduce such incidents, and making sure needles are sterilized can prevent infection.
Ultimately, all of the aforementioned issues—human responsibility, spiritual well-being, stress management, and so forth—relate to the religious issue of mind-body unity. Individuals generally know what is the right course of action—to exercise, to pass up on that donut or soft drink, to do a good deed for someone else, to not be overly concerned with oneself, and so forth. The issue really becomes one of actually doing those things, which may not be as pleasurable to the body in the short run. Thus, they are considered to be issues of mind-body unity: having the body do what the mind knows is the right course of action. That is, the mind is fundamental to good health, and good mind-body unity, centered on universal principles, will help promote good health.
Symptoms (headache, fever, high blood pressure, etc.) serve as warning signs of the deeper problem. In some cases, eliminating the symptoms—for example, a fever in a baby or sneezing caused by allergies—may be the only practical course available at the present time. However, generally the best course of action is not simply to eliminate or cover up the symptoms and then ignore the problem, but to treat the underlying causes and help the body improve its own health and ability to manage such stresses, now and in the future. The long-term solution may require making a lifestyle or environmental change to help in preventing disease.
Numerous healing methodologies are available. The Western method of taking drugs and employing surgery has been effective in many cases. However, many healing arts exist that are designed to treat the whole person and that address more than in conventional medicine. Alternative medicine includes naturopathy, chiropractic, ayurveda, homeopathy, and acupuncture. Holistic medicine strives to pay attention not just to the physical, but also the mental, emotional, and spiritual aspects of health, using such techniques as hypnosis and visualization. Natural medicine involves herbal remedies, diet, and water therapies. In addition to these alternative methods of healing, there is also the method of faith healing, which addresses the issue of a spiritual cause to disease.
This following list of common, well-known, or infamous diseases is neither complete nor authoritative. It is not intended to be a list of rare diseases, nor is it a list of mental disorders. This list includes both common names and technical names for diseases. A number of rare diseases may be present in this list.
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