How can you tell the difference between a rash or an allergic reaction?

Rash vs allergy: Reactions to foods, medications, insect (bee) stings often are generalized (all over), and usually are hives. Viruses and bacteria can trigger rashes that are typical for the infection (for ex., scarlet fever), but also hives. Trouble comes when on medications for an infection, and a rash or hive breaks out. Seek an experienced provider, allergist or dermatologist.