Fire corals (Millepora) are not true reef building corals but rather small animals with bright yellow-green to brown skeletal coverings that can be razor-sharp. The calcium carbonate exoskeleton is full of tiny pores through which tentacles covered with stinging cells (nematocysts) project. They grow in different sizes and shapes and are found in shallow tropical and subtropical waters.

When a victim brushes against the projecting tentacles, the nematocysts fire and inject venom causing immediate burning or stinging pain. Rubbing the area will worsen the envenomation and subsequent pain. The area of contact may develop pinpoint red lesions. Edema, blistering, and urticaria are common. The blisters may develop into pustules. Resolution of lesions may take 3-7 days without treatment. Hyperpigmentation of the affected area may last 4-8 weeks.

Nausea and vomiting have been reported but are not common.