An emotional support animal (ESA) is a type of assistance animal that alleviates a symptom or effect of a person's disability. An emotional support animal is not a pet and is generally not restricted by species. An emotional support animal differs from a service animal. Service animals are trained to perform specific tasks (such as helping a blind person navigate), while emotional support animals receive no specific training, nor even, necessarily, any training at all. (It therefore stands that in the setting of mental illness, whether or not the animal is a "service animal" vs. an emotional support animal would hinge on whether or not it is formally trained to do something specific to mitigate the mental illness.) Any animal that provides support, well-being, comfort, or aid, to an individual through companionship, unconditional positive regard, and affection may be regarded as an emotional support animal. In the U.S., people with emotional or mental disabilities can be exempted from certain federal housing and travel rules if they own an emotional support animal. To receive that exemption, they must meet the federal definition of disabled, and they must present a letter from a certified healthcare provider, stating that the animal provides emotional support that alleviates one or more of the symptoms or effects of the disability. Emotional support animals are typically cats and dogs, but may be members of other animal species. In relation to whether or not an emotional support animal should be allowed in a rental property, it is thus necessary to perform an individualized assessment of the specific assistance animal to determine if it poses a direct threat of harm or would cause substantial property damage, and not to assume that an animal is excluded based upon breed or species. Although a wild or exotic animal that poses an increase risk of disease or potential attack upon other people may potentially be excluded, courts have recognized species including guinea pigs and miniature horses as emotional support animals. Laws and regulations that allow service animals to be taken into businesses or onto aircraft may give the service provider discretion to deny admission to unusual service animals. For example, under the Air Carrier Access Act, airlines are never required to accommodate unusual animals such as ferrets, rodents, snakes and other reptiles, or spiders within the passenger cabin of an airplane. In 2018, Delta Air Lines banned pit bulls and similar breeds of dogs from the passenger compartment of their aircraft as emotional support animals, after a pit bull traveling as an emotional support animal bit two employees. Most airlines will allow emotional support animals, with proper documentation from a veterinarian and/or mental health counselor, and small animals such as cats and dogs can be held on the passenger's lap during the flight. There is no requirement under federal law for emotional support animals to wear a tag, harness, or clothing of any type indicating they are emotional support animals. Emotional support animals do not need to have any special training. There are no training requirements for emotional support animals. Emotional support animals typically have no training beyond what would be expected for the same type of animal. Emotional support animals need not perform any tasks other than what a pet of the same species would perform, and may display unwanted behaviors, such as defecating or urinating in inappropriate places, growling and barking at people, or biting them. Both poorly trained emotional support animals and poorly trained pets that are being fraudulently passed off as emotional support animals represent a threat to the health, safety, and function of both people and trained service animals. To qualify for an emotional support animal in the US, its owner must have an emotional or mental disability that is certified by a mental health professional such as a psychiatrist, psychologist, or ot