Mental Retardation

Mental retardation is a condition in which people have significantly below average mental functioning (an intelligence quotient or IQ of 70-75 or less compared to the normal average of 100), causing problems with everyday A trio of winners celebrate their victories at a Special Olympics meet in North Carolina. The Special Olympics were founded in 1968 to provide children and adults with mental retardation continuing opportunities to train and compete in athletic events.

Metabolic Disease

Nearly 100 years ago, a British scientist named Archibald Garrod suggested that people actually could inherit genetic (je-NE-tik) information that causes problems with the body's metabolism. A gene is the unit of heredity that carries physical characteristics from parent to child.

Mononucleosis, Infectious

*virus (VY-rus) is a tiny infectious agent that lacks an independent metabolism (me-TAB-o-liz-um) and can only reproduce within the cells it infects.

Motion Sickness

Jon and his dad were very excited about their upcoming deep sea-fishing trip off the Florida coast. Dad's friend Bob had a boat and knew the right spots to look for fish.

Mouth Cancer

Mouth cancer usually begins in the tissues that make up the lips, tongue, or cheek lining, but it also can affect the gums, the floor or the roof of the mouth, or the salivary glands*. In almost all cases, it is caused by the use of substances that irritate the mucous membranes* in the mouth: spit tobacco (also called chewing tobacco or snuff), cigarettes, cigars, pipes, or alcohol.

Multiple Personality Disorder (Dissociative Identity Disorder)

Sybil Dorsett was a 22-year-old college student who had amnesia*. She also had terrible headaches and sometimes could not see, as if she were blind.

Multiple Sclerosis

Jacqueline Du Pré was born in England in 1945. On her fifth birthday, her parents gave her a cello, and she started lessons the next year.


The mumps virus is spread through the air from an infected person to another person and incubates in the body for 14 to 24 days until symptoms appear. A person who has been infected by the mumps virus can spread the disease to others about a week before any symptoms appear and for about 2 weeks after.

Muscular Dystrophy

The term "muscular dystrophy" actually includes a group of disorders that affect different muscles in the body, and that may range from mild to severe. Although all of the muscular dystrophies are known to be caused by genes*, the way in which the genes produce the disorder is as yet only partly understood.


Kate noticed she was squinting when she needed to see the blackboard from the back of her classroom. Squinting helped bring the words into focus.


Polly will always remember fifth grade because she missed so much of it. She missed one week of school because she had strep throat, a sore throat caused by streptococcal bacteria.


During the summer when Sally was seven years old, she stopped eating much of anything at meals, lay around the house all day, and seemed too tired to do her chores or play with friends. At first Sally's mother thought Sally was just being lazy, but when Sally's whole body began to look puffy, especially her eyes, ankles, and abdomen, her mother began to worry.


The most common form, called neurofibromatosis type 1 or NF-1, mainly affects nerves in the skin, producing soft nodules or bumps. Neurofibromatosis type 2 (NF-2) is a very rare disorder that affects the auditory nerves responsible for hearing and balance.


Most nosebleeds occur in children and older people and last for a short time. In rare cases, nosebleeds can be associated with other illnesses.


Karen was a large baby who grew into a very large child. By the time she was 12 years old, Karen was 60 pounds overweight.

Obsessive-Compulsive Disorder

In the 1997 movie As Good As It Gets, actor Jack Nicholson portrays a character who does many things that seem odd. He feels as if he has to eat lunch at the same table each day, and he always brings his own plastic uten-sils.

Osgood-Schlalter Disease

Danny's twelfth birthday marked the beginning of soccer season, a 4-inch growth spurt, and a lot of pain in his right knee. When Danny's coach asked him why he was limping, Danny told him that his knee hurt, especially when climbing stairs, kneeling, or jumping.


Kyle stepped on a nail on his first day of vacation at the lake. It punctured the bottom of his foot and hurt pretty badly.


Bone consists of two layers: a compact outer layer, called cortical bone, and a porous inner layer, called spongy (or cancellous) bone. Osteoporosis weakens mostly bones with a large percentage of spongy bone.