Allergic purpura is mostly caused by exposure to infections, medications, or foods, leading to skin and mucous membrane ecchymoses, which are essentially petechiae and a manifestation of vasculitis. It falls under autoimmune diseases, not genetic diseases, and thus is not contagious. Bacterial, viral, mycoplasmal, or parasitic infections are the most common causes. Secondarily, medications and foods can also trigger allergic purpura, such as high-protein foods like seafood and milk. Therefore, if there are symptoms like ecchymoses on the lower extremities accompanied by joint pain, or signs of blood in the stool or black stool, it is necessary to complete routine blood tests, erythrocyte sedimentation rate, and autoimmune antibody tests to rule out other autoimmune diseases. During the acute phase, it is advisable to rest in bed, eat a light diet, avoid infection, and use corticosteroids for anti-allergy treatment if necessary.