Knock-knees
A deformity of the legs in which the knees are abnormally close together and the ankles are spread too far apart; known a genu valgus.

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Laceration
A cut in the skin; treatment depends on the location, depth and severity of the wound, but stitches (sutures) may be required for proper wound healing.

Laryngitis
Inflammation of the "voice box" (larynx) causing hoarseness or loss of voice; usually caused by a viral infection that goes away without treatment.

Lazy eye
Strabismus; "crossed eye"; "weak eye"; "straying eye"; misalignment of the eyes; when one eye turns either inward or outward relative to the other eye; treatment may include surgery or patching of the unaffected eye to promote focusing of the lazy eye; when not treated early in childhood, can have significant effects on both vision (amblyopia) and physical appearance.

Leukemia
Cancer of the white blood cells marked by anemia, bleeding, infections, organ enlargement and/or swollen lymph glands; some forms of leukemia, especially acute lymphocytic leukemia (ALL) which is the most common form seen in children, are now curable with chemotherapy and radiation therapy.

Lice (head lice)
Small parasites (Pediculosis capitis) that usually make their home in hairy areas of the body where they lay small white eggs (nits) that stick to the bottom of the hair shafts; infestation is called "pediculosis"; a very common, highly contagious condition in young children; head lice are a nuisance, but they are harmless.

Lockjaw
Trismus; prolonged tightening of the jaw muscles causing the jaws to be clenched shut; often an early sign of tetanus; rarely seen in developed countries because of successful tetanus immunization programs ( the "T" in DTaP vaccine).

Lupus (systemic lupus erythematosus)
A disease marked by inflammation in the tissues that connect and support all parts of the body (connective tissue); usually chronic and found more often in females than males; can affect any part of the body including skin, kidneys, joints and heart.

Lyme disease
A disease caused by the bacterium, Borrelia burgdorferi; early symptoms include a skin rash that can be distinctive (erythema migrans), fever, tiredness, headaches, joint aches and, sometimes, nervous system problems; late disease may be marked by arthritis, brain or nerve problems; spread to people by infected ticks; can be treated with antibiotics.

Lymph glands
Lymph nodes; accumulations of tissue along lymph vessels throughout the body; become enlarged in response to a variety of conditions including infections and some cancers; produce immune products including lymphocytes.

Lymphadenopathy
Enlarged lymph nodes; may be seen in a variety of disease states including infections and cancer.

Lymphocytes
Small white blood cells that help the body defend itself from foreign substances like organisms capable of causing infection.

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Malaria
An infectious disease caused by microscopic parasites that live in red blood cells; marked by recurring cycles of fever, chills and sweating; spread to people by the bite of an infected mosquito; can be treated with antimalarial drugs.

Measles (rubeola)
A highly contagious, potentially life-threatening vaccine-preventable viral infection marked by fever, cough, runny nose, conjunctivitis, a red rash and distinctive spots inside the lining of the cheek; can be prevented by measles vaccine (one of the "M"s in MMR vaccine).

Melanoma
Cancer of the pigment (melanin) secreting cells of the skin; may start in moles, the eye, or anywhere on the skin or mucous membranes; more common in adults than children.

Meningitis
Inflammation of the membranes lining the brain and spinal cord; marked by headache, stiff neck, fever, vomiting and encephalopathy; usually caused by bacteria or viruses; bacterial meningitis is a medical emergency that must be treated promptly because it can cause serious brain damage or death.

Microcephaly
An abnormally small head.

Migraine
A specific kind of periodic headache, usually one-sided and accompanied by nausea, vomiting, vertigo and visual disturbances; often runs in families.

Mittelschmerz
Middle pain; abdominal pain occurring in the middle of the menstrual cycle at the time of ovulation.

Mole
Nevus; An overgrowth of pigment-forming cells resulting in a slightly raised, sometimes hairy growth on the skin; generally present at birth or develops early in life; most are harmless, but some kinds of moles need to be removed because they tend to become cancerous.

Molluscum contagiosum
A harmless infection of the skin marked by scattered small bumps that may be dented in the center; caused by a poxvirus; can resolve on its own over a period of several weeks, but treatment is advised to prevent spread.

Mongolian spots
Harmless bluish-black superficial coloring of the skin, usually on the buttocks and back, but can be found anywhere; most commonly seen on dark-skinned infants; generally fade away over time and are not associated with any problems.

Mononucleosis (infectious mononucleosis)
A viral infection associated with sore throat, swelling of the lymph nodes, tiredness, fever and, sometimes, rash; may last several weeks; caused by the Epstein-Barr virus(EBV); no specific treatment is available.

Mucous membranes
Linings of body cavities including the mouth, nose, sinuses, eyelids, and genital, urinary and gastrointestinal tracts.

Mucus
A thick slimy substance secreted by the inside lining of the nose, breathing and other passages (mucous membranes); mucus secretion is often increased during infection.

Mumps
A mild vaccine-preventable viral illness that typically causes painful swelling of the saliva-producing glands (parotids) of the face and neck; occasionally affects the pancreas, testicles, ovaries or brain; prevented by mumps vaccine (one of the "M"s in MMR vaccine).

Muscular dystrophy
A group of inherited conditions marked by gradual and progressive muscle wasting and weakness; different forms range from mild to severe.

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Nephritis
Inflammation of the tissues of the kidney, usually associated with blood in the urine and high blood pressure; may be caused by autoimmune disorders, drugs, toxins or infections.

Nephrotic syndrome
A clinical syndrome characterized by the kidney's excreting excessive amounts of protein into the urine, causing low protein levels in the blood and swelling (edema) of the body; usually responds to treatment.

Nevus
A mole.