Infectious diseases are caused by bacteria, viruses, protozoa, fungi, and rickettsia that invade the body of a susceptible host and cause an illness.
They are transmitted from one animal to another by contact with infected urine, feces, and other bodily secretions, or by inhaling pathogen-laden droplets. They may also be acquired by contact with spores in the soil that enter the body through the respiratory tract or a break in the skin. A few are sexually transmitted. Although pathogens exist everywhere in the environment, only a few cause infection. Fewer still are contagious.
Many infectious diseases are species- specific. For example, a dog cannot catch a disease that is specific to a horse, and vice versa. Other infectious diseases are not species-specific, and are capable of causing disease in many animals, including humans.
In instances where a disease is zoonotic,public health considerations are discussed. Many infectious agents are able to survive for long periods outside the host animal. This knowledge is important in determining how to contain the spread of infection. For many diseases, the best way to prevent them is by vaccination.