Answer: Oxygen

During the mid-20th century, there was a veritable epidemic of blindness among premature babies. Thousands of babies in just the U.S. alone, including the very well known blind performer Stevie Wonder, were injured by a treatment meant to help them.

Historically, premature babies were automatically put on oxygen therapy. After all, their lungs were underdeveloped, oxygen is critical to life, and who doesn’t benefit from an extra dose of it? While increased oxygenation of the blood certainly helps everyone from performance athletes to people recovering from injuries, it’s not so great for premature babies. It turns out that the elevated oxygen levels in the premature babies damaged the tiny arteries in their eyes, triggered runaway and wild growth of new blood vessels, and led to irreversible damage as the unchecked death and growth of the blood vessels detached the retina and filled the eye with fibrous tissue.

Studies in the early 1950s established that routine oxygen supplementation caused the eye damage and oxygen supplementation was immediately reduced in frequency and duration. When only premature babies in true respiratory distress were given oxygen, the rate of blindness among premature babies dropped immediately.