Fire ants are stinging ants found worldwide. They live in large colonies in the ground and typically feed on plants, seeds, and small insects. Unlike most other ants that bite to kill their prey, fire ants bite to hold onto their prey, then inject a toxic alkaloid venom with their posterior sting. They can sting multiple times. Most species of fire ants do not come into contact with humans and, therefore, are not of medical significance. The exception is the red imported fire ant, Solenopsis invicta (RIFA). Native to South America, this ant has now spread to the southern and southwestern United States, eastern Australia, the Philippines, Taiwan, and China. In areas where red imported fire ants live, up to 60% of the human population is stung annually.

The sting of a fire ant causes an immediate painful, burning sensation (hence the name "fire" ants). Soon after, most victims develop an IgE-mediated wheal and flare reaction near the bite site. Within the next 24 hours, vesicles develop that are initially filled with clear fluid. The fluid then becomes cloudy, leaving a typical fire ant lesion that is an umbilicated, sterile pustule on a red, erythematous base. These pustules may persist up to 10 days but will eventually rupture, form a crust, and sometimes leave a scar. Secondary infection can occur when pustules rupture.

Fire ant venom is mainly composed of piperidine alkaloids, which cause histamine release and necrosis of human skin. A very small percent (0.1%) of the venom is composed of allergens that can induce an IgE-mediated reaction and may be cross-reactive with bee and wasp venom. Allergic reactions are common, but anaphylaxis is rare (< 1%-2%). Death from massive stinging events (5 000-10 000 stings) can occur, typically in victims who are incapacitated, such as invalids or intoxicated individuals.

Anaphylactic reactions cause diffuse urticaria, pruritus, angioedema, bronchoconstriction, respiratory distress, hypotension, loss of consciousness, and cardiac arrhythmias. Typically, onset of life-threatening, anaphylactic signs occur within 10 minutes of the sting.

Systemic reactions including seizures, peripheral neuropathy, serum sickness, nephropathy, and worsening of cardiopulmonary disease have been reported in individuals with numerous stings or those with manifestations of anaphylaxis.