These rather diminutive, tree-huggers resemble the teddy bears given to children, and unlike many of their larger cousins only eat veggies. Apart from being small, koala bears are also adorable, peaceful, fluffy, and riddled with deal breaking sexually transmitted diseases. Although koala bears may seem to have a sweet demeanor, if you see one in the wild don't dare touch it. The cuts, bruises, and bites these little guys can deliver will be the least of your worries. Koala bears are notorious for peeing intentionally on predators, and it's the bacteria in this pee that raises concern. Many koalas carry a strand of the STD Chlamydia, and their pee can transmit the disease to others it touches. Contracting an STD from an animal is probably not something you want hanging over your head for life, so better to stay away from koala bears altogether.