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• A to Z: Allergy

  With allergies, the immune system overreacts and tries to “fight” allergens, which are ordinary things that cause an allergic reaction.

• A to Z: Allergy, Drug

  Normally medicines help us treat disease, but in people with drug allergies, taking certain medicines can cause the immune system to react and harm the body.

• A to Z: Alopecia Areata

  Alopecia areata is a condition that causes hair loss on the scalp and sometimes elsewhere on the body.

• A to Z: Alpha Thalassemia

  Learn about genetic disorders and diseases and conditions that affect the blood.

• A to Z: Amebiasis

  Amebiasis is an intestinal illness. People get it after eating, drinking, or touching something that’s contaminated with a tiny parasite called Entamoeba histolytica.

• A to Z: Amenorrhea

  Amenorrhea is when a girl is not getting her period.

• A to Z: Amenorrhea, Primary

  Primary amenorrhea is when a girl does not start her period by age 15 or 3 years after she begins puberty.

• A to Z: Amenorrhea, Secondary

  Secondary amenorrhea is when a girl who had been getting her period stops menstruating for more than 6 months or for more than three of her usual cycles.

• A to Z: Anal Fissure

  An anal fissure is a cut or tear in the lining of the anus.

• A to Z: Anaphylaxis

  Anaphylaxis is a sudden, life-threatening reaction that is usually caused by an allergy.