Final answer:
A disease normally found in other animals but that starts infecting humans is a zoonosis.
Step-by-step explanation:
A disease that is normally found in animals but also starts infecting humans is called a zoonosis. These diseases are transmitted from vertebrate animals to humans, and sometimes the term encompasses diseases transmitted by all animals. For example, rabies is a known zoonotic disease that can spread from animals to humans. Other examples include yellow fever, transmitted through mosquito bites, and Rocky Mountain spotted fever, which is transmitted via ticks.
Epidemiologists are critical in understanding and preventing these diseases, as they investigate outbreaks much like detectives solve crimes. They track the emergence of new infectious diseases to human populations, many of which are zoonotic diseases.
Veterinarians and scientists also research the transmission mechanisms of avian influenza and other serious strains like the H5N1, a pathogenic avian flu virus, increasing our understanding of how such viruses can cross species. The study of these diseases falls under the field of epidemiology, and is central to public health research and the prevention of pandemics.