Seabather's eruption, also known as 'sea lice', is a rash that occurs after exposure to seawater that contains certain species of the larval form of the thimble jellyfish (Linuche unguiculata). The jellyfish larvae contain venom-filled barbs that fire their venom into the skin when the larvae become trapped under a bathing suit. This is because most modern swim suits are made of fine-meshed fabrics, for example Nylon and Lycra, that work like plankton nets, letting the water through but straining out small planktonic animals. The rash appears 4 to 48 hours after exposure to infested seawater. The symptoms are more common in children than adults and may include, in addition to the rash: fever, sore throat, abdominal pain, headache, cough, and diarrhea. Both the rash and the other symptoms generally resolve in 1 to 2 weeks.

Outbreaks of seabather's eruption in a given area appear to be caused by changes in the temperature of the water, the direction of the wind, and shifts in currents during the reproductive season of the jellyfish. Outbreaks can occur between March and August and normally peak in May and June.

The surface area of a bathing suit may increase the area over which stings can occur. Female bathers wearing one-piece suits and children and adults wearing T-shirts in the water can increase the risk of a more severe reaction. It is not unusual to see of 200 or more stings under a person's bathing suit.

Treatment includes the administration of an antihistamine for the itching and the application of 0.5% hydrocortisone cream to the rash areas. One may also want to consult a doctor.

To prevent the eruption, shower as soon as possible after exiting the ocean. Remove and wash your bathing suit, separately, as soon as possible. Machine dry, do not air dry, the bathing suit to kill any remaining larva. Nude swimming, which is popular in many tourist areas of Jamaica, can also help prevent seabather's eruption. Also consider wearing thong or G-string style swimwear and/or swimwear made with more open types of fabric such as fishnet as a preventative measure.

Jellyfish in both the planula and ephyra stages of life are considered to be 'sea lice' and can cause an eruption.