The complex process of damage and resistance to damage that occurs in animal organisms due to internal or external pathogenic factors and adverse effects, manifested as local, organ, systemic, or systemic morphological changes and/or functional impairments. In a healthy state, animals maintain a dynamic balance with their environment, and the structure and function of the body are in a normal state; Diseases disrupt this balance. In this process, if the damage exceeds the body's defense ability, the disease will worsen and even lead to death; On the contrary, the disease *, body recovery, and occasional adverse consequences may occur.

1. According to the classification of disease nature:

Infectious diseases: Their pathogens include viruses, bacteria, rickettsia, chlamydia, molds, fungi, and other microorganisms. The characteristics are: ① Each infectious disease is caused by a specific microorganism, and the host spectrum varies in width. Such as and are respectively caused by swine fever virus and anthrax bacteria; Swine fever can only infect pigs, while anthrax can infect almost all mammals, including humans. ② It is contagious. Pathogenic microorganisms can be transmitted from infected animals to healthy animals through direct contact (such as licking, biting, mating, etc.), indirect contact (such as air, drinking water, fertilized semen, etc.), dead vectors (such as livestock utensils, contaminated surgical instruments, etc.), and live vectors (such as arthropods, rodents, birds, humans, etc.), causing the same disease. ③ Pathological changes and clinical symptoms manifested by invasion of certain organs, systems, or even the whole body. ④ After being infected, animals often produce immune biological reactions (immune and allergic reactions), which humans can use to create various methods for the diagnosis, treatment, and prevention of infectious diseases.

Parasitic diseases: Parasites mainly include three categories: protozoa, worms, and arthropods. The first two are mostly internal parasites, while the majority of the latter are external parasites. Parasites often have a long developmental period and a complex life history, and some need to complete their development in one or even several host bodies. Most parasites have a fixed end host, which can be transmitted through direct contact (such as scabies mites, Trypanosoma cruzi, hookworm filamentous larvae, hemochromatosis cercariae), ingestion of soil, water or feed containing infectious eggs, larvae or egg sacs (such as roundworms, roundworms, nematodes), as well as external parasites such as ticks and flies as vectors (such as hemochromatids).

Common diseases mainly include three categories: internal medicine, surgery, and obstetric diseases. Internal medicine diseases include neurological, cardiovascular, hematopoietic organ, endocrine, muscular, skeletal and other systems, as well as nutritional metabolism, poisoning, genetics, immunity, and diseases of young animals. Their causes and manifestations are diverse. Surgical diseases mainly include trauma, etc. Obstetric diseases can be divided into pregnancy diseases (stillbirths, etc.), childbirth diseases, postpartum diseases, breast diseases, newborn and young animal diseases, etc. based on their occurrence period.

But the above classification is not. Some diseases caused by protozoa, such as,, and others, are very similar to infectious diseases in terms of transmission, prevalence, and manifestation, and some scholars also classify them as infectious diseases. The nail fistula caused by the invasion of the horseneck ligament by the coiled tail silk worm is both a parasitic disease and a surgical disease. Surgical removal is considered a surgical disease, while non-surgical removal is considered an internal medicine disease. As for the branch of livestock diseases established due to the emphasis on the cultivation of young livestock and the anatomical and physiological characteristics of young livestock, its content includes infectious diseases, parasitic diseases, and common diseases. Classified for ease of description and application, there are no boundaries that cannot be exceeded.

2. Classification based on the number of animals simultaneously affected:

It can be divided into two types: cluster onset and sporadic onset. Generally, infectious diseases, parasitic diseases, poisoning, and nutritional deficiency diseases are mostly cluster onset, but there are exceptions. For example, although tetanus is an infectious disease, it requires the presence of Clostridium tetani in the deep to occur, so it is only sporadic. Some infectious diseases, such as toxoplasmosis and schistosomiasis, as well as some parasitic diseases, often manifest as latent infections in livestock herds, mostly sporadic and only occasionally occurring in groups. In mass diseases, they can be classified into local, epidemic, and pandemic diseases according to their epidemic patterns. Although common diseases are mostly sporadic, some poisoning diseases and nutrient deficiencies, especially trace element deficiency diseases, are often endemic due to their etiology being related to the characteristics of feed and soil in a certain region. In infectious and parasitic diseases, some are only endemic due to their pathogens (such as Bacillus anthracis, Clostridium difficile and their spores) often existing in the soil of a certain area, or their intermediate hosts (such as snails containing schistosomiasis cercariae, ticks carrying pear shaped worms) limited to a certain water area or region. But most highly infectious diseases such as swine fever and other infectious diseases often spread and occur simultaneously in large areas, known as epidemic diseases; Some of them can still spread rapidly across continents at the same time, constituting a pandemic or a worldwide outbreak.

2. Other categories:

Animal diseases can also be divided into local diseases and foreign diseases. In the context of frequent communication, well-developed tourism and trade, foreign diseases often enter the country through mosquitoes, flies, fleas, lice, bed bugs, cockroaches, mice and accompanying animals in various modes of transportation, imported livestock and wild animals, frozen semen and embryos, as well as livestock products and other goods. Preventing imported animal diseases has become an important task for border and port quarantine.