In 1984, Marvel Comics created a new nemesis for Spider-Man. The character would be a symbiote, inspired by what parasitologists call the weaker of two organisms inhabiting the same space. The weaker organism can draw life from the stronger, and in the most dramatic cases it siphons off its host’s nutrients before the host realizes what’s happening. The symbiote survives, but the host is seriously weakened.
Once Marvel Comics had a new symbiotic character, that character needed a host. It struck a bargain with another character named Eddie Brock: the symbiote would give Brock its power in return for Brock’s life energy. But of course symbiotes from outer space cannot be trusted. Once the symbiote had inhabited Brock, it absorbed his life energy and morphed into the evil Venom.