Sudden swelling and pain in the belly of the thumb is commonly due to the following causes: first, nail fungus, bacterial infection caused by nail clipping injury or ingrown nails, causing swelling, pain, local tenderness, and elevated skin temperature in the thumb. Second, purulent dactylitis, the etiology includes stabbing with sharp objects or undetected minor injuries, clinical manifestations are initially mild pain, progressing faster, local tension, patients may experience throbbing pain, painful symptoms are aggravated at night, local redness, swelling, heat and pain, local blood flow is obstructed as the condition progresses, skin can appear pale, when a fluctuating sensation is formed, suggesting abscess, radiographs show osteomyelitis of the end phalanges or the presence of X-rays show osteomyelitis of the end phalanges or the presence of dead bone, and routine blood tests show an increased white blood cell count and an increased percentage of neutrophils.