Bacteria are prokaryotic unicellular organisms which are in general either Gram positive, Gram negative or acid-fast in nature. Various groups of these bacteria produce different infectious diseases in the susceptible animals. Among these, the Gram positive bacteria such as Staphylococcus, Streptococcus, Listeria, Clostridium, Bacillus etc. are well known causes of diseases like mastitis, strangles, pyoderma, listeriosis, black quarter, enterotoxaemia, anthrax etc. Similarly, Gram negative bacteria produce variety of infections in animals viz ., haemorrhagic septicaemia caused by Pasteurella spp., brucellosis caused by Brucella spp., salmonellosis caused by Salmonella spp. etc .

The acellular entities grow and replicate only in other living cells. They possess either DNA or RNA as genetic material. Accordingly, these viruses can be grouped into DNA viruses and RNA viruses. Viruses produce variety of infectious diseases in the livestock. Major viral diseases affecting large ruminants like cattle and buffalo include Foot-and-mouth disease, Infectious bovine rhinotracheitis, Lumpy skin disease, buffalopox etc. Similarly, Peste des Petits Ruminants (PPR), Bluetongue, goat pox, sheep pox, contagious ecthyma etc. are the major viral diseases of sheep and goats. Major viral diseases affecting swine population include Classical swine fever, Porcine respiratory and reproductive syndrome, African swine fever and porcine circoviral infections.

These are the infectious agents that are devoid of genetic material and possess the proteinaceous capsid only. These proteinaceous agents produce certain diseases in livestock e.g. bovine spongiform encephalopathy in cattle, scrapie in sheep etc.

Animal often suffer from fungal infections. The diseases produced by the fungal pathogens are known as mycoses. These fungal infections may be categorized into cutaneous mycoses e.g. Dermatophytosis; subcutaneous mycoses e.g. Rhinosporidiosis, Sporotrichosis; Systemic mycoses e.g. Histoplasmosis, Blastomycosis, Coccidioidomycosis, Paracoccidioidomycosis etc. Dermatophytosis (Ringworm) infection is produced by dermatophytes, a group of keratiniphilic fungi, in the keratinized layer of the skin. Rhinosporidiosis is caused by Rhinosporidium seeberi characterized by formation of large polyps or wart like tissue growth in the nasal cavity of the farm animals. Histoplasmosis is a pulmonary infections produced by a dimorphic fungi, Histoplsma capsulatum. Aspergillosis is the most prevalent fungal infection produced by Aspergillus fumigatus. It affects the respiratory tract of farm animal. Apart from these fungal infections, mycotic mastitis and mycotic abortions are few of the important infections affecting the farm animals, especially the milch and pregnant animals, respectively. Mycotoxins are produced by group of certain fungi in the feed of farm animals. Consumption of such mycotoxin contaminated feed often results in mycotoxicoses.

Farm animals suffer from parasitic infections to a wide extent. These may be categorized into endoparasitic infections and ectoparasitic infections. Among the parasitic infections, Amphistomiasis caused by Amphistomes, Fascioliasis caused by Fasciola spp. as well as Theileriosis caused by Theileria spp. are few important parasitic diseases affecting livestock, especially cattle.

The immunological disorders, immunologic diseases or immune-mediated diseases are result of impairment of certain immunological component or process in the body of the animal. Such compromised immunological system of the animal may predispose the animal to variety of the other health problems. e.g. immunodeficiency diseases, autoimmune diseases and hypersensitivity reactions.

The genetic disorders or genetic anomalies are the result of certain inherited abnormalities in the genetic makeup of the animal, like stable inherited mutations in certain genes in the animal results disturbance in the normal functioning of that particular gene or gene clusters.

The metabolic diseases usually occur, when animal is unable to meet the energy need for the essential metabolic processes of the body. e.g. acidosis, ketosis, Downer cow syndrome, milk fever etc. These diseases are in general acute and fatal in nature. Such diseases have to be attended by appropriate therapeutic interventions for restoration of essential body functions and survival of the animal. The diseases like Downer cow syndrome can be avoided by supplementation with certain essential dietary components like calcium and phosphorous during last trimester of gestation in pregnant animal.

Deficiency diseases occur due to shortage of certain essential nutrients viz. vitamins and minerals in the ration/feed of the livestock. e.g. hypocalcaemia in high yielding dairy herds due to vitamin D deficiency along with imbalance of calcium and phosphorous level in the feed; grass tetany due to magnesium deficiency. Such diseases can be overcome by supplementation of required nutrients in the feed or specifically, designing and feeding area-specific mineral mixtures to the farm animals.

It is the phenomenon of presence of malignant cells or cancerous cells in a particular tissue/organ of the body. These cells grow in an uncontrolled fashion and spread/invade locally to nearby cells/tissue or to the other part of the body. The process of spread of such malignancy from one part to other is called as metastasis. Such malignant cancerous growth is different from benign tumors, which are nothing but localised growth and do not spread like the former one. Besides, non-infectious causes of malignancy like chronic irritants and genetic predisposition, certain infectious agents also produce malignancy in the affected host. e.g. bovine leukaemia virus in cattle; avian leukosis virus and Marek's disease virus in poultry.