When working with dangerous animals, it's only logical that zoos and aquariums take whatever steps possible to mitigate the risk to their keepers. With some species, its means never sharing a space with the animal. When that's not possible, the danger can be lessened with training - both of the animals and the keepers. And, for venomous animals, when the worst happens, there is always that last safety net - antivenom.
Antivenom (sometimes also called antivenin) is a biological compound used to treat the bite or sting of a venomous animal. Essentially, it's an antidote to a venomous bite, obtained from the venom of that very species. Venom is collected from the species in question through a process called "milking," in which you basically get the snake to bite something that collects the venom. The venom is then diluted and then injected into another animal - a sheep, a rabbit, a horse. The venom is dilute enough that, instead of the traditional response (i.e., dying), the envenomated animal is able to produce antibodies to respond to the venom. These antibodies are then harvested, and can be used to treat future venomous bites in people.
It works a lot like a vaccine, in that a weakened initial infection is used to develop immunity for a more serious future infection. The major difference is that vaccines are often administered early to head off the outbreak of a disease. Antivenom is usually applied after the person has been bitten, and seeks to transfer the immunity from one organism to another.
An antivenom can be a monovalent (effective against one, single species) or a polyvalent (effective against several, usually closely related species). The bite of a king cobra, for instance, is treated with a monovalent, effective against king cobras and king cobras alone. On the other hand, the bites of a wide variety of North American pit-vipers - the several species of rattlesnakes, along with copperheads and cottonmouths - can be treated with CroFab, a polyvalent that was derived from antibodies harvested in sheep.
Antivenom has its disadvantages, to be sure. It's expensive to produce. It has a relatively short shelf-life. Some people have allergic reactions. And it can take a heck of a lot of it to treat a major snake bite. Its management is very complicated, leading some zoos to wash their hands of venomous species altogether, except maybe local native species, for which local hospitals will likely already have antivenom.
To help confront the challenges of dealing with antivenom, in 2006 the Association of Zoos and Aquariums and the American Association of Poison Control Centers collaborated to develop an Antivenom Index, showing what facilities have antivenoms for what species. In an emergency, doctors and zoo directors can contact the list and rush antivenom to where it is needed the most.
In one last quirk of venom protection, a small handful of animal handlers (none I've ever met, I admit) will actively envenomate themselves with small doses, trying to build up a natural immunity. The process is called Mithridatism, after Mithridates VI, a Greek king who lost his father to assassination by poisoning and didn't want to follow suite. According to legend, he began ingesting small amounts of poisonous substances in an effort to build up immunity to any toxins his rivals might slip into his food or drink. It has been theorized that Mithridatism is what allowed the Russian monk Rasputin to survive poisoning attempts; at any rate, it's been practiced by venomous handlers all over the world, from snake charmers to venom milkers. It's effectiveness is limited, and is not effective against all types of venom and poison.
That and it can also kill you in the end. Not a recommended method.