Zoonoses are infectious diseases of animals (usually vertebrates) that can naturally be transmitted to humans.

Major modern diseases such as Ebola virus disease and salmonellosis are zoonoses. HIV was a zoonotic disease transmitted to humans in the early part of the 20th century, though it has now evolved to a separate human-only disease. Most strains of influenza that infect humans are human diseases, although many strains of swine and bird flu are zoonoses; these viruses occasionally recombine with human strains of the flu and can cause pandemics such as the 1918 Spanish flu or the 2009 swine flu. "Taenia solium" infection is one of the neglected tropical diseases with public health and veterinary concern in endemic regions. Zoonoses can be caused by a range of disease pathogens such as viruses, bacteria, fungi and parasites; of 1,415 pathogens known to infect humans, 61% were zoonotic. Most human diseases originated in animals; however, only diseases that routinely involve animal to human transmission, like rabies, are considered direct zoonosis.

Zoonoses have different modes of transmission. In direct zoonosis the disease is directly transmitted from animals to humans through media such as air (influenza) or through bites and saliva (rabies). In contrast, transmission can also occur via an intermediate species (referred to as a vector), which carry the disease pathogen without getting infected. When humans infect animals, it is called reverse zoonosis or anthroponosis. The term is from Greek: ζῷον "zoon" "animal" and νόσος "nosos" "sickness".

Zoonotic transmission can occur in any context in which there is companionistic (pets), economic (farming, etc.), predatory (hunting, butchering or consuming wild game) or research contact with or consumption of animals, animal products, or animal derivatives (vaccines, etc.).

Spotted fever can be very difficult to diagnose in its early stages, and even experienced doctors who are familiar with the disease find it hard to detect.

People infected with "R. rickettsii" usually notice symptoms following an incubation period of one to two weeks after a tick bite. The early clinical presentation of Rocky Mountain spotted fever is nonspecific and may resemble a variety of other infectious and non-infectious diseases.

Initial symptoms:

- Fever

- Nausea

- Emesis (vomiting)

- Severe headache

- Muscle pain

- Lack of appetite

- Parotitis in some cases (somewhat rare)

Later signs and symptoms:

- Maculopapular rash

- Petechial rash

- Abdominal pain

- Joint pain

- Conjunctivitis

- Forgetfulness

The classic triad of findings for this disease are fever, rash, and history of tick bite. However, this combination is often not identified when the patient initially presents for care. The rash has a centripetal, or "inward" pattern of spread, meaning it begins at the extremities and courses towards the trunk.

The rash first appears two to five days after the onset of fever, and it is often quite subtle. Younger patients usually develop the rash earlier than older patients. Most often the rash begins as small, flat, pink, non-itchy spots (macules) on the wrists, forearms, and ankles. These spots turn pale when pressure is applied and eventually become raised on the skin. The characteristic red, spotted (petechial) rash of Rocky Mountain spotted fever is usually not seen until the sixth day or later after onset of symptoms, but this type of rash occurs in only 35 to 60% of patients with Rocky Mountain spotted fever. The rash involves the palms or soles in as many as 80% of the patients. However, this distribution may not occur until later on in the course of the disease. As many as 15 percent of patients may never develop a rash.

Rocky Mountain spotted fever (RMSF), also known as blue disease, is the most lethal and most frequently reported rickettsial illness in the United States. It has been diagnosed throughout the Americas. Some synonyms for Rocky Mountain spotted fever in other countries include “tick typhus,” “Tobia fever” (Colombia), “São Paulo fever” or “"febre maculosa"” (Brazil), and “"fiebre manchada"” (Mexico). It is distinct from the viral tick-borne infection, Colorado tick fever. The disease is caused by "Rickettsia rickettsii", a species of bacterium that is spread to humans by "Dermacentor" ticks. Initial signs and symptoms of the disease include sudden onset of fever, headache, and muscle pain, followed by development of rash. The disease can be difficult to diagnose in the early stages, and without prompt and appropriate treatment it can be fatal.

The name “Rocky Mountain spotted fever” is something of a misnomer. The disease was first identified in the Rocky Mountain region, but beginning in the 1930s, medical researchers realized that it occurred in many other areas of the United States. It is now recognized that the disease is broadly distributed throughout the contiguous United States and occurs as far north as Canada and as far south as Central America and parts of South America. Between 1981 and 1996, the disease was reported from every state of the United States except for Hawaii, Vermont, Maine, and Alaska.

Rocky Mountain spotted fever remains a serious and potentially life-threatening infectious disease. Despite the availability of effective treatment and advances in medical care, approximately three to five percent of patients who become ill with Rocky Mountain spotted fever die from the infection. However, effective antibiotic therapy has dramatically reduced the number of deaths caused by Rocky Mountain spotted fever. Before the discovery of tetracycline and chloramphenicol during the latter 1940s, as many as 30% of those infected with "R. rickettsii" died.

The symptoms of an infection depend on the type of disease. Some signs of infection affect the whole body generally, such as fatigue, loss of appetite, weight loss, fevers, night sweats, chills, aches and pains. Others are specific to individual body parts, such as skin rashes, coughing, or a runny nose.

In certain cases, infectious diseases may be asymptomatic for much or even all of their course in a given host. In the latter case, the disease may only be defined as a "disease" (which by definition means an illness) in hosts who secondarily become ill after contact with an asymptomatic carrier. An infection is not synonymous with an infectious disease, as some infections do not cause illness in a host.

Infections can be classified by the anatomic location or organ system infected, including:

- Urinary tract infection

- Skin infection

- Respiratory tract infection

- Odontogenic infection (an infection that originates within a tooth or in the closely surrounding tissues)

- Vaginal infections

- Intra-amniotic infection

In addition, locations of inflammation where infection is the most common cause include pneumonia, meningitis and salpingitis.

Foodborne illness (also foodborne disease and colloquially referred to as food poisoning) is any illness resulting from the food spoilage of contaminated food, pathogenic bacteria, viruses, or parasites that contaminate food, as well as toxins such as poisonous mushrooms and various species of beans that have not been boiled for at least 10 minutes.

Symptoms vary depending on the cause, and are described below in this article. A few broad generalizations can be made, e.g.: The incubation period ranges from hours to days, depending on the cause and on how much was consumed. The incubation period tends to cause sufferers to not associate the symptoms with the item consumed, and so to cause sufferers to attribute the symptoms to gastroenteritis for example.

Symptoms often include vomiting, fever, and aches, and may include diarrhea. Bouts of vomiting can be repeated with an extended delay in between, because even if infected food was eliminated from the stomach in the first bout, microbes (if applicable) can pass through the stomach into the intestine and begin to multiply. Some types of microbes stay in the intestine, some produce a toxin that is absorbed into the bloodstream, and some can directly invade deeper body tissues.

Foodborne illness usually arises from improper handling, preparation, or food storage. Good hygiene practices before, during, and after food preparation can reduce the chances of contracting an illness. There is a consensus in the public health community that regular hand-washing is one of the most effective defenses against the spread of foodborne illness. The action of monitoring food to ensure that it will not cause foodborne illness is known as food safety. Foodborne disease can also be caused by a large variety of toxins that affect the environment.

Furthermore, foodborne illness can be caused by pesticides or medicines in food and natural toxic substances such as poisonous mushrooms or reef fish.

Hepatitis E is a viral hepatitis (liver inflammation) caused by infection with a virus called hepatitis E virus. It is one of five known human hepatitis viruses: A, B, C, D, and E. HEV is a positive-sense, single-stranded, nonenveloped, RNA icosahedral virus; HEV has a fecal-oral transmission route. Infection with this virus was first documented in 1955 during an outbreak in New Delhi, India. A preventive vaccine (HEV 239) is approved for use in China.

Although hepatitis E often causes an acute and self-limiting infection (the virus usually resolves itself and the individual recovers) with low mortality rates in the western world, it bears a high risk of developing chronic hepatitis in immunocompromised patients with substantial mortality rates. Organ transplant recipients who receive immunosuppressive medication to prevent rejection are thought to be the main population at risk for chronic hepatitis E. Furthermore, in healthy individuals during the duration of the infection, the disease severely impairs a person’s ability to work, care for family members, and perform other daily activities. Hepatitis E occasionally develops into an acute, severe liver disease, and is fatal in about 2% of all cases. Clinically, it is comparable to hepatitis A, but in pregnant women, the disease is more often severe and is associated with a clinical syndrome called fulminant liver failure. Pregnant women, especially those in the third trimester, suffer an elevated mortality rate from the disease of around 20%.

Hepatitis E newly infected about 28 million people in 2013.

Infection with hepatitis E virus can also lead to problems in other organs. For some of these reported conditions the relationship is tenuous, but for several neurological and blood conditions the relationship appears causal:

- Acute pancreatitis

- Guillain-Barré syndrome (acute limb weakness due to nerve involvement) and neuralgic amyotrophy (arm and shoulder weakness)

- Hemolytic anemia in people with the hereditary risk factor glucose-6-phosphate dehydrogenase deficiency (G6PD deficiency)

- Glomerulonephritis with nephrotic syndrome and/or cryoglobulinemia

- Mixed cryoglobulinemia, where antibodies in the bloodstream react inappropriately at low temperatures

- Severe thrombocytopenia (low platelet count in the blood) which confers a risk of dangerous bleeding

Spondylodiscitis is a combination of discitis (inflammation of one or more intervertebral disc spaces) and spondylitis (inflammation of one or more vertebrae), the latter generally involving the areas adjacent to the intervertebral disc space.

Spondylodiscitis is the most common complication of sepsis or local infection, usually in the form of an abscess. The main causative organisms are staphylococci and Mycobacterium tuberculosis, but potential organisms include a large number of bacteria, fungi, zoonoses. Spondylodiscitis frequently develops in immunocompromised individuals, such as by a cancer, infection, or by immunosuppressive drugs used for organ transplantations.

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.

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.

Perhaps the strongest psychological model put forward to explain animal hoarding is obsessive–compulsive disorder (OCD). An overwhelming sense of responsibility for something is characteristic of people with OCD, who then take unrealistic measures to fulfill their perceived duty. Animal hoarders often feel a strong sense of responsibility to take care of and protect animals, and their solution—that of acquiring as many animals as they possibly can—is unrealistic. Further, the hoarding of inanimate objects, practiced by a majority of animal hoarders, is a fairly common occurrence in people with OCD. These connections between animal hoarding and obsessive–compulsive disorder suggest that OCD may be a useful model in explaining animal hoarding behavior. However, this theory has also been refuted by some; Dr. Akimitsu Yokoyama theorizes that animal hoarding could be explained using Asperger syndrome.

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.