The first cases appeared unrelated: a rash from a brush with poison oak, perhaps, or maybe an allergic reaction to insect bites. But as more children at the Stinson Beach Preschool kept itching, the teachers realized something else was going on: the school had an outbreak of scabies, a skin infection caused by microscopic, slow-moving itch mites burrowing under the skin. First identified by Aristotle as “lice in the flesh,” scabies is one of the most common skin disorders among kids globally, affecting 130 million people at any given time, particularly in tropical and subtropical areas. It spreads person-to-person primarily through direct, prolonged physical contact. After finding a new host, the male parasite fertilizes the female on the skin’s surface before she tunnels into the skin, laying two or three eggs a day across the burrow in the skin for up to six weeks before she dies. Symptoms are usually undetectable for the first month, but after the eggs hatch and more burrowing or movement commences, itching is common, usually in the body’s crevices: between fingers and toes or near wrists, elbows, armpits or the waist or genital area. Scabies are often treated with an insecticide, permethrin cream, which is applied once from the neck down in adults and over the entire body for young children for eight to 12 hours. The West Marin Pharmacy quickly sold out of the few tubes they had in stock and placed and sold out two more full orders to meet demand, said pharmacist and owner Zsuzsanna Biran. But after the long weekend, no one came in to fill a prescription. To prevent further infestation, members in exposed households should apply the ointment; wash all articles of clothing, sheets and towels in hot water; and bag any materials that can’t be washed, like stuffed animals, for a week or two. For an alternative treatment, the West Marin Pharmacy has also prepared a topical ointment made from sulfur, Ms. Biran said. Susan Tacherra, the director and head teacher at the preschool, did not respond to calls at home and at the preschool or emails. So far there have been no reported cases at the Bolinas Children’s Center or in other districts, administrators said. One confirmed case was reported at the elementary school campus in Stinson Beach, and information was sent home to the whole school alerting parents of warning signs and treatments, said Elaine Doss, the Bolinas-Stinson School principal.