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immune system — a complex system in the body that recognizes and responds to potentially harmful substances, like infections, in order to protect the body.

incest — When two people who are too closely related to marry have sex, such as a parent and a child or two first cousins.

indigestion — also called dyspepsia. Indigestion is a common problem that causes a vague feeling of abdominal discomfort after meals. Symptoms also can include an uncomfortable fullness, belching, bloating, and nausea. It may be triggered by eating particular foods, after drinking wine or carbonated drinks, or by eating too fast or overeating.

infertility — A condition in which a couple has problems conceiving, or getting pregnant, after one year of regular sexual intercourse without using any birth control methods. Infertility can be caused by a problem with the man or the woman, or both.

inflammation — used to describe an area on the body that is swollen, red, hot, and in pain.

inflammatory bowel disease — long-lasting problems that cause irritation and ulcers in the gastrointestinal tract. The most common disorders are ulcerative colitis and Crohn's disease.

inhaled medicines — administered by having the user breath in the substance.

insecticides — chemicals used to control or kill insects.

insomnia — not being able to sleep.

insulin — one of many hormones that helps the body turn the food we eat into energy and helps store energy to be used later. People with diabetes mellitus, a condition in which the body does not make enough insulin, might need to inject themselves with insulin to help their bodies’ cells work properly.

interferon — a group of proteins with a carbohydrate component, which is produced by different cell types in response to an exposure of a virus, bacterium, or parasite, that prevents replication (of the virus, bacterium, or parasite) in newly infected cells.

interstitial cystitis — a long-lasting condition also known as painful bladder syndrome or frequency-urgency-dysuria syndrome. The wall of the bladder becomes inflamed or irritated, which affects the amount of urine the bladder can hold and causes scarring, stiffening, and bleeding in the bladder.

intestines — also known as the bowels, or the long, tube-like organ in the human body that completes digestion or the breaking down of food. They consist of the small intestine and the large intestine.

intimidation — To make someone fearful in order to make them do what another person wants them to do.

intrauterine device — a small device that is placed inside a woman's uterus by a health care provider, which prevents pregnancy by changing the environment of the uterus (or womb).

intravenous analgesic — An analgesic is a drug that relieves pain. During labor, a woman can be given pain-relieving drugs intravenously (through a tube inserted into her vein).

inverted nipple — a nipple that retracts, rather than protrudes when the areola is compressed.

ischemia — decrease in the blood supply to a an organ, tissue, or other part caused by the narrowing or blockage of the blood vessels.