Acne is a common skin disease characterized by pimples on the face, chest, and back. It occurs when the pores of the skin become clogged with oil, dead skin cells, and bacteria.


Addiction is a dependence on a substance, such as the drug heroin, or a type of behavior, such as gambling. The dependence is so strong that it may seem as if the person is unable to break away from the dependence.


AIDS is the abbreviation for acquired immune deficiency syndrome. The disease is caused by a virus known as the human immunodeficiency virus, or HIV.


Alcoholism is the popular term for two disorders: alcohol abuse and alcohol dependence. The key element of these disorders is that a person's use of alcohol has repeatedly caused problems in his or her life.


Allergies are abnormal reactions of the immune system to substances that are otherwise harmless.

Alzheimer's Disease

Alzheimer's (pronounced ALTS-hih-merz) disease (AD) is the most common form of dementia (pronounced dih-MEN-sha). Dementia is a disease of the nervous system characterized by loss of certain mental abilities.

Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS; pronounced ay-MY-eh-TRO-fik LA-ter-el skler-OH-sis) is a neurodegenerative disease. A neurodegenerative disease is one in which nerve cells are damaged or killed.


Anemia is a condition characterized by abnormally low levels of red blood cells.

Anorexia Nervosa

Anorexia nervosa is an eating disorder that occurs primarily among girls and women. It is characterized by a fear of gaining weight, self-starvation, and a distorted view of body image.


Appendicitis (pronounced uh-pen-di-SIE-tis) is the inflammation of the appendix, a worm-shaped pouch near the beginning of the large intestine. The appendix has no known function in the body, but it can become diseased.


Arthritis means inflammation of a joint. There are more than one hundred different forms of arthritis.


Asthma (pronounced AZ-muh) is a chronic (long-lasting) inflammatory disease of the airways in the human body. The inflammation causes the airways to narrow from time to time.


Atherosclerosis (pronounced ath-uh-ro-skluh-RO-siss) is the build-up of a waxy deposit on the inside of blood vessels. Atherosclerosis is a form of arteriosclerosis (pronounced ar-tir-ee-o-skluh-RO-siss).

Athlete's Foot

Athlete's foot is a common fungus infection that occurs between the toes. The skin becomes itchy and sore and cracks and peels away.

Attention-Deficit/Hyperactivity Disorder

Attention-deficit/hyperactivity disorder (ADHD) is a condition that shows up most commonly in boys and girls after puberty begins. Puberty is the period in life when a person's sex hormones become active.


Autism is a severe disorder of brain function. It is marked by problems with social contact, intelligence, and use of language.

Autoimmune Disorders

Autoimmune disorders are conditions in which a person's immune system (the network of organs, tissues, cells, and chemicals whose job it is to protect the body from foreign invaders, like bacteria and viruses) attacks the body's own cells. As cells are killed, tissues begin to die off.

Bipolar Disorder

Bipolar disorder is a mental condition that usually involves extreme mood swings. A person with the condition may feel happy and excited at one moment and depressed the next.

Breast Cancer

Breast cancer is cancer of the breast. Cancer (see cancer entry) is a disease in which cells begin to grow rapidly and out of control.


Bronchitis (pronounced brong-KIE-tis) is an inflammation of the air passages between the nose and the lungs. It affects the trachea (pronounced TRAY-kee-uh), or windpipe, and the bronchi.

Bulimia Nervosa

Bulimia (pronounced bu-LIM-ee-uh) nervosa is a serious and sometimes life-threatening eating disorder that affects primarily young women. Bulimics (people who have bulimia) go through cycles of binging and purging.

Burns and Scalds

Burns are injuries to tissues caused by heat, friction, electricity, radiation, or chemicals. Scalds are a type of burn caused by a hot liquid or steam.


Cancer is not just one disease, but a group of almost one hundred diseases. These diseases have two common characteristics.

Carpal Tunnel Syndrome

Carpal tunnel syndrome (CTS) is a disorder caused by pressure on the median nerve in the wrist. Numbness and tingling are characteristic symptoms of the disorder.


A cataract is a cloudiness in the lens of the eye, which is normally clear and transparent. The cloudiness caused by a cataract may eventually lead to decreased vision and blindness.

Cerebral Aneurysm

A cerebral aneurysm (pronounced an-yuh-RIH-zim) occurs at a weak spot in an artery that supplies blood to the brain. Once weakened, the artery wall bulges outward and fills with blood.

Cerebral Palsy

Cerebral palsy (CP) is the name given to a collection of movement disorders. Movement disorders are conditions in which a person's muscles do not respond normally.


Chickenpox, also called varicella (pronounced VAR-ih-SEL-uh), is a common and very infectious childhood disease, which sometimes affects adults. Chickenpox produces an itchy rash with blisters that lasts about a week.

Chronic Fatigue Syndrome

Chronic fatigue syndrome (CFS) is a condition that causes extreme tiredness. People with CFS are so tired that they are unable to carry on normal activities for a period of at least six months.

Color Blindness

Color blindness is a condition in which people have mild to severe difficulty identifying colors. Color blind people may not be able to recognize various shades of colors and, in some cases, cannot recognize colors at all.

Colorectal Cancer

Colorectal (pronounced KO-lo-REK-tul) cancer is cancer of the colon or rectum. The colon is the lowest portion of the large intestine and is the last part of the digestive system through which food passes.

Common Cold

The common cold is a viral infection of the upper respiratory system. The upper respiratory system includes the nose, throat, sinuses, eustachian (pronounced yoo-STA-shuhn) tubes, trachea (pronounced TRAY-kee-uh), larynx, and bronchial tubes.


Concussion is a change in mental status caused by trauma (shock). It is accompanied by confusion, loss of memory, and, sometimes, loss of consciousness.


Conjunctivitis (pronounced kuhn-junk-tuh-VIE-tis) is an inflammation (redness) of the conjunctiva. The conjunctiva is the membrane that lines the white part of the eye and the underside of the eyelid.

Creutzfeldt-Jakob Disease

Creutzfeldt-Jakob disease (CJD; pronounced KROITS-felt-YAH-kop) is a highly infectious disease that affects the nervous system.

Crohn's Disease

Crohn's disease (pronounced krohnz) is a type of inflammatory bowel disease. It results in swelling of the intestinal tract.

Cystic Fibrosis

Cystic fibrosis (CF) is a hereditary disease that affects the lungs, digestive system, sweat glands, and male reproductive organs.


Cystitis (pronounced sis-TIE-tess) is inflammation of the bladder. The condition is often associated with inflammation of other structures adjoining the bladder.

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