Dictionary

oropharynx The part of the throat that is at the back of the mouth, in contrast to the nasopharynx (the part of the throat that is behind the nose).

orphan disease A disease that has not been “adopted” by the pharmaceutical industry because it provides little financial incentive for the private sector to make and market new medications to treat or prevent it. An orphan disease may be a rare disease (according to US criteria, a disease that affects fewer than 200,000 people) or…

orphan drug A drug that is designed to treat or prevent an orphan disease. See also orphan disease.

ORS Oral rehydration solution.

opportunistic infection An infection that is not typically seen in healthy people, but occurs because a person’s immune system is weakened. Opportunistic infections are a particular danger for people with immunodeficiency, such as AIDS, or for those receiving immunosuppressive drugs such as chemotherapy for cancer. The HIV virus itself does not cause death, but the…

opportunistic microorganism A bacterium, virus, or fungus that takes advantage of certain opportunities to cause disease (opportunistic conditions). Opportunistic microorganisms are often ones that can lie dormant in body tissues for many years, such as the human herpesviruses, or that are extremely common but usually cause no symptoms of illness. When the immune system cannot…

oppositional defiant disorder A behavior disorder of children and teens characterized by an ongoing pattern (for at least 6 months) of uncooperative, defiant, and hostile behavior toward authority figures that seriously disrupts normal day-to-day functioning. Symptoms include anger, defiance, temper tantrums, revenge-seeking behaviors, and hateful or mean talking. Abbreviated ODD. The cause of ODD is…

optic Having to do with vision.

optic nerve The second cranial nerve, which connects the eye to the brain. The optic nerve carries the impulses that are formed by the retina—the nerve layer that lines the back of the eye, senses light, and creates impulses. These impulses are dispatched through the optic nerve to the brain, which interprets them as images….

optic nerve pathways The course of the chemical and electrical impulse from light stimulating the retina as it passes from the optic nerve to the vision center of the brain. The left and right branches of the optic nerves join behind the eyes, just in front of the pituitary gland, to form a cross-shaped structure…

optic neuroma A rare benign tumor of the optic nerve.

optician A specialist in fitting eyeglasses and making lenses to correct vision problems. An optometrist performs eye examinations and writes prescriptions for corrective lenses; an optician fills that prescription.