Zoophilia is a paraphilia involving a sexual fixation on non-human animals. Bestiality is cross-species sexual activity between human and non-human animals. The terms are often used interchangeably, but some researchers make a distinction between the attraction (zoophilia) and the act (bestiality).

Although sex with animals is not outlawed in some countries, in most countries, bestiality is illegal under animal abuse laws or laws dealing with buggery or crimes against nature.

Paraphilia (previously known as sexual perversion and sexual deviation) is the experience of intense sexual arousal to atypical objects, situations, fantasies, behaviors, or individuals. Such attraction may be labeled sexual fetishism. No consensus has been found for any precise border between unusual sexual interests and paraphilic ones. There is debate over which, if any, of the paraphilias should be listed in diagnostic manuals, such as the "Diagnostic and Statistical Manual of Mental Disorders" (DSM) or the International Classification of Diseases (ICD).

The number and taxonomy of paraphilia is under debate; one source lists as many as 549 types of paraphilia. The DSM-5 has specific listings for eight paraphilic disorders. Several sub-classifications of the paraphilias have been proposed, and some argue that a fully dimensional, spectrum or complaint-oriented approach would better reflect the evidence.

Many terms have been used to describe atypical sexual interests, and there remains debate regarding technical accuracy and perceptions of stigma. Sexologist John Money popularized the term "paraphilia" as a non-pejorative designation for unusual sexual interests. Money described paraphilia as "a sexuoerotic embellishment of, or alternative to the official, ideological norm." Psychiatrist Glen Gabbard writes that despite efforts by Stekel and Money, "the term "paraphilia" remains pejorative in most circumstances."

Coinage of the term "paraphilia" ("paraphilie") has been credited to Friedrich Salomon Krauss in 1903, and it entered the English language in 1913, in reference to Krauss by urologist William J. Robinson. It was used with some regularity by Wilhelm Stekel in the 1920s. The term comes from the Greek παρά ("para") "beside" and φιλία ("-philia") "friendship, love".

In the late 19th century, psychologists and psychiatrists started to categorize various paraphilias as they wanted a more descriptive system than the legal and religious constructs of sodomy and perversion. Before the introduction of the term "paraphilia" in the DSM-III (1980), the term "sexual deviation" was used to refer to paraphilias in the first two editions of the manual. In 1981, an article published in "American Journal of Psychiatry" described paraphilia as "recurrent, intense sexually arousing fantasies, sexual urges, or behaviors generally involving:

- Non-human objects

- The suffering or humiliation of oneself or one's partner

- Children

- Non-consenting persons

Formicophilia, a form of zoophilia, is the sexual interest in being crawled upon or nibbled by insects, such as ants, or other small creatures. This paraphilia often involves the application of insects to the genitals, but other areas of the body may also be the focus. The desired effect may be a tickling, stinging or in the case of slugs, slimy sensation, or the infliction of psychological distress on another person. The term was coined by Ratnin Dewaraja and John Money in 1986 from the Latin "formica" (ant) + the Greek "philia" (love).

Three key terms commonly used in regards to the subject — "zoophilia", "bestiality", and "zoosexuality" — are often used somewhat interchangeably. Some researchers distinguish between zoophilia (as a persistent sexual interest in animals) and bestiality (as sexual acts with animals), because bestiality is often not driven by a sexual preference for animals. Some studies have found that a preference for animals is rare among people who engage in sexual contact with animals. Furthermore, some zoophiles report that they have never had sexual contact with an animal. People with zoophilia are known as "zoophiles", though also sometimes as "zoosexuals", or even very simply "zoos". Zooerasty, sodomy, and zooerastia are other terms closely related to the subject but are less synonymous with the former terms, and are seldom used. "Bestiosexuality" was discussed briefly by Allen (1979), but never became widely established. Ernest Bornemann (1990, cited by Rosenbauer, 1997) coined the separate term "zoosadism" for those who derive pleasure – sexual or otherwise – from inflicting pain on animals. Zoosadism specifically is one member of the Macdonald triad of precursors to sociopathic behavior.

In the first reported case study, the patient had started keeping ants in a cupboard of his room as a hobby when he was nine. At this age he enjoyed "the ticklish feeling" of the ants crawling on his legs and thighs. At age ten, he had a sexual relationship with another boy, and was beaten when his father discovered this. By the age of 13 or 14, he had added snails and cockroaches to his collection, and was becoming increasingly preoccupied with it. He had begun masturbating while the ants crawled on his legs. At age 28, he was masturbating several times a week while cockroaches crawled on his thighs and testicles, and snails crawled over his nipples and penis. Sometimes he would hold a frog against his penis and enjoy the vibrations as it tried to escape. The patient was disgusted by his habit, but derived no pleasure from normal sexual activities. John Money suggested that the paraphilia developed as an alternative outlet after his normal sexual expression became associated with the trauma of his father's punishment.

Another case was described in 2012. At the age of 14, the patient saw an ice-cream stick covered with ants, and wondered how it would feel if his penis was in place of the stick. He began letting ants crawl on his genitals, especially fire ants, a practice he found sexually exciting and that continued into adulthood. The patient was socially and intellectually competent. He was also attracted to dogs and goats.

Animal hoarding is keeping a higher-than-usual number of animals as domestic pets without ability to properly house or care for them, while at the same time denying this inability. Compulsive hoarding can be characterized as a symptom of mental disorder rather than deliberate cruelty towards animals. Hoarders are deeply attached to their pets and find it extremely difficult to let the pets go. They typically cannot comprehend that they are harming their pets by failing to provide them with proper care. Hoarders tend to believe that they provide the right amount of care for them. The American Society for the Prevention of Cruelty to Animals provides a "Hoarding Prevention Team", which works with hoarders to help them attain a manageable and healthy number of pets.

An animal hoarder keeps an unusually large number of pets for their premises, and fails to care for them properly. A hoarder is distinguished from an animal breeder, who would have numerous animals as the central component of their business; this distinction can be problematic, however, as some hoarders are former breeders who have ceased selling and caring for their animals, while others will claim to be breeders as a psychological defense mechanism, or in hopes of forestalling intervention. Gary Patronek, director of the Center for Animals and Public Policy at Tufts University, defines hoarding as the "pathological human behavior that involves a compulsive need to obtain and control animals, coupled with a failure to recognize their suffering". According to another study, the distinguishing feature is that a hoarder "fails to provide the animals with adequate food, water, sanitation, and veterinary care, and... is in denial about this inability to provide adequate care." Along with other compulsive hoarding behaviors, it is linked in the DSM-IV to obsessive compulsive disorder and obsessive compulsive personality disorder. The DSM-5 includes a diagnosis of hoarding disorder.

Alternatively, animal hoarding could be related to addiction, dementia, or even focal delusion.

The number of animals involved alone is not a determinative factor in identifying hoarding. Instead, the issue is the owner’s inability to provide care for the animals and the owner’s refusal to acknowledge that both the animals and the household are deteriorating. For instance, in one animal hoarding case, 11 cats were seized from a trailer. The deputy police officer, who had severe congestion and was on sinus medication at the time of the investigation, testified that the trailer smelled so bad of urine and feces that she had a hard time staying in there for more than a few minutes. The deputy further testified that she couldn't step anywhere in the trailer without stepping on fresh, old, or smeared fecal matter, and that even the stove and sink were filled with feline waste. Yet, a Canadian woman, who died leaving 100 properly fed, spayed, neutered, vaccinated, and groomed cats, was not considered an animal hoarder because her animals were properly cared for.

The Hoarding of Animals Research Consortium (HARC) identifies the following characteristics as common to all hoarders:

- Accumulation of numerous animals, which has overwhelmed that person’s ability to provide even minimal standards of nutrition, sanitation, and veterinary care;

- Failure to acknowledge the deteriorating condition of the animals (including disease, starvation, and even death) and the household environment (severe overcrowding, very unsanitary conditions); and−

- Failure to recognize the negative effect of the collection on their own health and well-being, and on that of other household members.

Compulsive hoarding can be characterized as a symptom of mental disorder rather than deliberate cruelty towards animals. Hoarders are deeply attached to their pets and find it extremely difficult to let the pets go. They typically cannot comprehend that they are harming their pets by failing to provide them with proper care. Hoarders tend to believe that they provide the right amount of care for their pets. The American Society for the Prevention of Cruelty to Animals provides a "Hoarding Prevention Team", which works with hoarders to help them attain a manageable and healthy number of pets.

Aujeszky's disease, usually called pseudorabies in the United States, is a viral disease in swine that has been endemic in most parts of the world. It is caused by "Suid herpesvirus 1" (SuHV1). Aujeszky's disease is considered to be the most economically important viral disease of swine in areas where hog cholera has been eradicated. Other mammals, such as humans, cattle, sheep, goats, cats, dogs, and raccoons, are also susceptible. The disease is usually fatal in these animal species bar humans.

The term "pseudorabies" is found inappropriate by many people, as SuHV1 is a herpesvirus and not related to the rabies virus.

Research on SuHV1 in pigs has pioneered animal disease control with genetically modified vaccines. SuHV1 is now used in model studies of basic processes during lytic herpesvirus infection, and for unravelling molecular mechanisms of herpesvirus neurotropism.

Respiratory infection is usually asymptomatic in pigs more than 2 months old, but it can cause abortion, high mortality in piglets, and coughing, sneezing, fever, constipation, depression, seizures, ataxia, circling, and excess salivation in piglets and mature pigs. Mortality in piglets less than one month of age is close to 100%, but it is less than 10% in pigs between one and six months of age. Pregnant swine can reabsorb their litters or deliver mummified, stillborn, or weakened piglets. In cattle (see next section), symptoms include intense itching followed by neurological signs and death. In dogs, symptoms include intense itching, jaw and pharyngeal paralysis, howling, and death Any infected secondary host generally only lives two to three days.

Genital infection appears to have been common in a great part of the 20th century in many European countries in swine herds, where boars from boar centres were used for natural service of sows or gilts. This disease manifestation has always been asymptomatic in affected pigs, and presence of the infection on a farm was detected only because of cases in cattle showing pruritus on the hindquarters (vaginal infection, see below).

In susceptible animals other than swine, infection is usually fatal, and the affected animals most often show intense pruritus in a skin area.

Pruritus in Aujeszky's disease is considered a phantom sensation, and virus has never been found at the site of pruritus.