Cellulitis: This is a common bacterial skin infection that can cause redness, swelling, and warmth of the skin. It often occurs without any visible injury, and the hand is a common site due to its exposure and frequent minor traumas that may not be recalled.

Insect Bite or Sting: Certain insect bites or stings can cause significant localized swelling and redness, especially if there is an allergic reaction.
Contact Dermatitis: An allergic reaction or irritation from substances like soap, detergents, or certain plants can lead to swelling and redness of the hand.
Gout or Pseudogout: Although more commonly associated with the feet, these conditions can cause sudden, severe swelling and redness in the hands, especially if there is a history of similar episodes.

Necrotizing Fasciitis: A severe bacterial infection that can cause swelling, redness, and pain, often without an obvious source of infection. It is a medical emergency due to its rapid progression and potential for severe complications or death.
Septic Arthritis: Bacterial infection within a joint can cause swelling, redness, and severe pain. Prompt diagnosis and treatment are crucial to prevent joint destruction.
Deep Vein Thrombosis (DVT): Although less common in the hand, DVT can cause swelling and redness. It is a critical diagnosis to consider due to the risk of pulmonary embolism.

Sarcoidosis: A condition that can cause inflammation in various parts of the body, including the skin, and might present with swelling and redness of the hand.
Erythromelalgia: A disorder that causes burning pain, heat, and redness in the hands and feet, which could be mistaken for an infectious or inflammatory cause.
Complex Regional Pain Syndrome (CRPS): A condition characterized by persistent burning pain and swelling, often following an injury, but can sometimes occur without a clear precipitant.