Answer: Fungus

Despite what the name might imply, ringworm is not caused by worms, nor was there ever any belief among doctors that ringworm infections were caused by actual worms.

Ringworm is, in fact, a fungal infection of the skin that is called such because it typically presents itself as a red, scaly, and circular patch of skin that spreads outward, leaving an ever widening raised ring that looks like a red worm beneath the skin.

Infections are extremely common, especially among children, and up to 20 percent of the population may be infected at any given time. Thankfully, the condition is easily treated with over-the-counter topical anti-fungals (and in a fashion, much safer than the mercury, sulfur, iodine, and x-ray treatments used around the start of the 20th century).