Zoos usually try to recreate the animals natural habitat and coinhabitants (lacking the predator of course). They are experts and do mix species that live together in the same ecosystem. They are also trained to treat the animals if the become ill. Also the simulated habitats that are created at Zoos are usually much larger than a hobbiest has and makes it to where the individual species do not come in contact with one another. I'm not saying they never combine contact more so that it is less likely for them to do so.