Well it's just that they have very high concentrations of bacteria that would otherwise be harmless...until a 150lb lizard bites you. A wound full of these bacteria, left untreated, will cause sepsis and death within a few days. There is also some evidence that they produce weak venom, however that seems uncertain.

And of course they don't breathe fire. Would be really awesome if they did though, and I doubt that a zoo would miss out on having such a show. Just think of how many people would want to go see a fire breathing lizard!