People may become ill with ciguatera fish poisoning after eating certain contaminated fish. The ciguatera toxin is created by microalgae that naturally live in warm-water oceans, and this disease has been known for centuries. Fish eat the toxin-containing algae, and as larger fish eat smaller fish, the toxin becomes concentrated in the larger fish. Tropical and subtropical fish that may contain the toxin include amberjack, grouper, snapper, and barracuda. The presence of toxin in fish is unpredictable, and the frequency of ciguatera toxin illness in the United States is unknown. Ciguatera fish poisoning is probably underreported in the continental United States, since most cases are of short duration and it is not well known outside areas where it is common, such as the Caribbean and Hawaii. A cluster of cases in New York City from August 2010 to July 2011 affected 28 people. Ciguatera toxin–producing algae may be increasing because of warmer sea temperatures.