You probably know that a brown recluse spider is poisonous.
Well, brown recluse venom is both cytotoxic, meaning it can kill cells, and hemolytic, meaning it can destroy red blood cells. The extent of what this means if you're bitten by a brown recluse varies. However, the typical symptoms are as follows: within eight hours of being bitten, a bite victim experiences mild to severe pain and itching. At first, the skin around the bite reddens, next a blister forms, and soon after, necrosis occurs.
Necrosis is death to the surrounding tissue, yes, but not death to you. The tissue around the bite dies, blackening and eventually sloughing off to leave a large open sore that is slow to heal. Some people suffer more severe symptoms, such as the destruction of red blood cells and anemia. Signs that you're suffering a severe reaction to a brown recluse bite include fever and chills, a skin rash all over the body, and joint pain. If you show signs of having been bitten by a brown recluse, you should see a doctor immediately for treatment.
If you do get bit, you should try to calm down. Extra movement and excitement will only get your blood pumping faster and hence the venom circulating faster too.