What is the most dangerous creature on earth, i.e. which creature has caused the most total human deaths?

There are many deadly animals on this planet, but I've never been able to conclusively determine which has directly caused the most people to die. I'm not looking for secondary causation, such as malaria and yellow fever spread by mosquitoes, but immediate effect through poisoning, laceration, crushing or mutilating to the point of cessation of life. Please send me anything you can find, as well as sources to back up your claims!