Group A beta-hemolytic streptococci cause ecthyma. Risk factors include crowded living conditions, high temperature and humidity, and poor hygiene. Those with existing impetigo are at especially high risk. Preexisting tissue damage from dermatitis or insect bites, anemia, malnutrition, and a compromised immune system (diabetes, neutropenia, acquired immunodeficiency syndrome) may contribute to the development of this infection. A warm climate favors outbreaks of this infection, which usually occurs in late summer and early fall. The most common transmitters appear to be biting insects, such as mosquitoes and flies, with autoinoculation occurring through scratching.

A rare but serious complication of ecthyma is glomerulonephritis, caused by a nephritogenic strain of beta-hemolytic streptococci. Other complications of ecthyma include cellulitis, erysipelas, gangrene, lymphangitis, suppurative lymphadenitis, and bacteremia (rare). Scarlet fever may also occur. Permanent skin damage resulting from ecthyma is common. More extensive complications include staphylococcal scalded skin syndrome and toxic shock syndrome.

Ecthyma begins with a red-bordered vesicle or pustule that deepens into ulceration with a gray-yellow crust that is thicker than the crust seen in impetigo. Lesions may enlarge to 1¼ inches (3 cm) in diameter and may be painful. These lesions are usually found on the legs after a scratch or bug bite. Once the crust is gone, a shallow ulceration remains, with a raised and indurated margin. Autoinoculation can transmit ecthyma to other parts of the body, especially to sites that have been scratched open. Lymphadenopathy may occur.