The anaconda and other large constrictors are more than large and aggressive enough to kill and eat an adult human by strangulation or constriction, crushing the life from their prey and then swallowing it whole. Anacondas as small as 13 feet in length are large enough to eat humans whole and in fact prey on animals which are significantly larger and more dangerous.
People do not often make contact with large snakes because of their mutually exclusive habitats, but there are multiple records of snake attacks and predation. Human encroachment into snake territory can lead to violent encounters between the two species.
Snakes will attack most readily when they are in dire need of nutrition. Snakes rest for long periods between hunting and feeding and may behave boldly or recklessly in their hunting patterns if they go too long without sustenance, leading them to attack even strong and mobile organisms like humans which they perceive as threats.
Snakes are tremendously strong and an adult anaconda is far too powerful for even a healthy human to fight off. Their muscular bodies allow them to encircle and crush prey in a process known as constriction, squeezing the breath from prey so that it cannot fight back or escape.