Exotic animals can be problematic pets for a variety of reasons. Many exotic animals such as tigers are purchased when young, and such undomesticated animals become difficult to handle as they grow into adults, often leading owners to abandon them. Some animals, including reptiles, pose health risks to humans, who may be exposed to zoonotic pathogens. Some 93,000 human cases of salmonella occur yearly in the U.S. due to exposure to reptiles, and up to 9 in 10 macaque monkeys carry the herpes B virus, which can be fatal to humans. Other factors such as the illegal exotic animal trade and laws governing exotic animal ownership complicate exotic pet ownership.