Sharks like the great white can smell blood up to 3 miles away. According to National Geographic, sharks can smell one drop of blood in 25 gallons of water, 3 miles away.
Sharks like the great white shark are carnivores, meaning their diet consists of meat. Great whites grow to be about 15 feet long and can weigh up to 5,000 pounds. They have a gray body meant to blend in with the ocean bottom and are called "great white" sharks because of their white underbellies.
When one animal attacks another and blood is spilled, that blood can attract sharks for miles around. The shark's keen senses allow it to quickly hone in on the source of the blood and sharks often take a test bite to find out if a meat is suitable for eating. This test bite is what is likely what is happening when a shark bites a human.