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iatrogenic: Complications or poor effects caused by medical treatment.

IBD: Abbreviation for inflammatory bowel disease, a general term for two disorders—ulcerative colitis and Crohn's disease—that cause the intestines to become swollen and inflamed.

ICD: Abbreviation for implantable cardioverter defibrillator, a device implanted in the chest and connected to the heart that delivers a shock to stop a potentially deadly rhythm and restore a normal (sinus) rhythm.

ice pick headache: Stabbing, very intense headaches that come on suddenly and are very brief.

idiopathic: A condition or disease of unknown origin.

IgE: Abbreviation for immunoglobulin E, the substance responsible for most allergic reactions.

ileum: The final section of the small intestine.

iliopsoas muscles: Two muscles, running from the end of the spine to the thighbone, that are responsible for lifting the knee.

immediate hypersensitivity: A category of allergic reaction, triggered by specific allergens and involving IgE. The majority of allergic reactions to pollens, pets, dust, mold, food, and insect venom are of this type.

immobilize: To restrict the movement of a limb or other part of the body to help in healing.

immunity: The body's ability to resist infection and disease.

immunization: Injection of harmless bacteria or viruses to spur the body to produce antibodies so it can resist a particular disease.

immunoglobulin: Substances made by the immune system that attack foreign substances. Also known as antibodies.

immunoglobulin E: The substance responsible for most allergic reactions.

immunologically privileged site: A part of the body—such as the eye or ovaries—where the immune system isn't able to function because it may damage the tissue there.

immunosuppressant drug: Medication that stifles the body's immune response; often given after an organ transplant to prevent rejection of the new organ.

immunotherapy: Treating disease by enhancing or suppressing the body's immune system.

impacted: Something firmly fixed into place, such as a wisdom tooth.

impaction: A mass of hardened feces blocking the rectum or colon.

impaired fasting glucose: Blood sugar levels that are higher than normal but not yet diabetic. This term is used when the high blood sugar levels are found with a fasting plasma glucose test.

impaired glucose tolerance: Blood sugar levels that are higher than normal but not yet diabetic. This term is used when the high blood sugar levels are found with an oral glucose tolerance test.

impedance hearing testing: A test that sends sound waves to the eardrum to determine if a problem in the middle ear is causing hearing loss.

implantable cardioverter defibrillator: A device implanted in the chest and connected to the heart that delivers a shock to stop a potentially deadly rhythm and restore a normal (sinus) rhythm.

impotence: The inability of the penis to become firm or to stay firm enough to have sexual intercourse.

in situ: Latin for in place.

inactivated vaccines: Vaccines containing microbes that have been killed, and, therefore, are unable to cause disease.

incision: A cut made into the skin or an organ during surgery.

incontinence: Involuntary passing of urine or feces.

incubation period: The time between when a person is exposed to an infection and when symptoms appear.

infarct: An area of dead tissue caused by insufficient blood supply.

infarction: The death of tissue due to a lack of blood.

infection: The growth of harmful organisms that can cause disease, such as bacteria, in the body.

infectious arthritis: Arthritis caused by harmful organisms such as bacteria.

inferior myocardial infarction: Heart attack involving the back part of the heart-muscle wall.

inferior vena cava: The large vein that carries blood from the lower half of the body to the heart.

infiltrating cancer: A cancer that has spread from where it first developed into surrounding tissue.

inflammation: The body's reaction to injury or infection. It is characterized by swelling, heat, redness, and pain.

inflammatory bowel disease: A general term for two disorders, ulcerative colitis and Crohn's disease, that cause the intestines to become swollen and inflamed. Often referred to as IBD.

infusion: The slow injection of a fluid into a vein or tissues.

ingrown toenail: A condition in which the side of a toenail pierces the skin, causing pain, swelling, and sometimes infection.

inhibitory neurochemical: A chemical that stops the transmission of a message from one nerve cell to another.

injectable fillers: Substances injected into the skin to fill in wrinkles or add plumpness to the lips.

injection: Inserting fluids, such as medications, into the body by means of a hollow needle and syringe.

innate immunity: The body's basic defenses against disease or infection that are present from birth.

inner ear: The deepest part of the ear, consisting of the cochlea and the labyrinth.

inoperable: A condition that cannot be treated by surgery.

insomnia: The inability to fall asleep or remain asleep long enough to feel rested.

insulin: A hormone made by the pancreas that controls the amount of glucose (sugar) in the blood.

insulin resistance: A condition in which the body produces insulin but can't use it properly. This leads to diabetes.

insulin-dependent diabetes: Now called type 1 diabetes. It occurs when the body doesn't produce enough insulin to control blood sugar levels properly.

intense pulsed light: A device that emits broadband light to improve the look of the skin or remove unwanted hair.

intensity: In exercise, a measure of how hard the body is working. Cues like breathing, talking, and sweating help measure intensity through perceived exertion.

interferons: Proteins made by the body to protect against viruses, bacteria, and other harmful agents.

interleukins: A group of substances that act as messengers in the immune system.

intermediate-density lipoprotein: A type of lipoprotein. It consists of remnants of very-low-density lipoprotein (VLDL) that eventually turn into low-density lipoprotein (LDL).

intermittent claudication: Pain and cramping in the legs during exercise that is caused by narrowed or blocked arteries. Also known as peripheral artery disease of the legs.

international unit: An internationally accepted amount of a substance based on its biological activity or effect; used as a measurement for fat-soluble vitamins.

interpersonal therapy: Short-term talk therapy focused on identifying and addressing problems in current relationships and building social skills.

intervention: A planned, often group, meeting with a person with addiction, with the aim of overcoming denial and inducing the individual to seek treatment.

intervention study: A study that compares one group that receives a medication or other therapy (an intervention) and another group does not (controls).

intervertebral disk: One of the small, shock-absorbing cushions located between the vertebrae of the spine.

intervertebral foramen: The opening between vertebrae through which a spinal nerve exits the spinal column (plural: foramina).

intolerance: An adverse reaction that may have similar symptoms to an allergic reaction but does not engage the immune system, and thus is not an allergy.

intracerebral hemorrhage: A hemorrhagic stroke caused by the rupture of a blood vessel and subsequent bleeding into the brain tissue.

intraocular lens: A small artificial lens permanently fixed inside the eye to replace the natural lens during cataract surgery.

intrinsic sphincter deficiency: Inability of the urinary sphincter to close completely.

iodides: Salt compounds containing iodine that are used to control severe hyperthyroidism in special circumstances. They work by decreasing the thyroid gland's production and secretion of thyroid hormone.

iris: The colored ring in front of the lens that controls the size of the pupil and how much light enters the eye.

irritants: Substances such as tobacco, wood smoke, perfumes, and others that cause allergy-like symptoms, although the response is not an allergic reaction.

ischemia: Inadequate blood supply to an organ or part of the body.

ischemic heart disease: The most common form of heart disease, in which narrowed or blocked coronary arteries have difficulty supplying sections of the heart muscle with the blood they need (ischemia).

ischemic stroke: A stroke caused by an interruption in the flow of blood to the brain; almost always caused by a blood clot blocking a blood vessel.

islets of Langerhans: Clusters of hormone-producing cells, including alpha and beta cells, that appear throughout the pancreas.

isolated systolic hypertension: A form of high blood pressure (hypertension) characterized by elevated systolic blood pressure and normal diastolic pressure.

isometric: An action in which a muscle generates force but does not contract or extend enough to move a joint, such as when pushing against an immovable object.

isotonic: An actions in which a muscle generates force by contracting or lengthening to move an attached joint through its range of motion, such as lifting a dumbbell from knee-height to shoulder-height.

IU: Commonly used abbreviation for international unit, an internationally accepted amount of a substance based on its biological activity or effect; used as a measurement for fat-soluble vitamins.