But that led us to think about the other human ailments animals can become afflicted with. And they are many. We're not talking about diseases that we induce for experimental purposes through genetic modification, from cancer to rats with Alzheimer's--that's a different story entirely. Symptoms like depression, however, or obesity, are ailments animals can contract on their own (or with some help from humans). These are illnesses we may not associate with other members of the animal kingdom.