PARASITE—An organism that depends upon another organism (host) for its nutrients and protection, usually harming the host in the process.
Infectious diseases and those caused by parasites or malnutrition are not the only challenges to health we have faced. The ancients recognized epilepsy and called it “the sacred disease,” believing that the seizures of epileptics were caused by gods or demons. Babylonian documents describe the symptoms of epilepsy, as do Greek and Roman medical texts. Cancer too was known and feared by our ancestors, who attempted to heal these diseases with remedies both natural and magical. Eventually doctors and scientists turned to the study of animals to help them understand the mysteries of anatomy and the riddles of disease.
So why do scientists study animals to understand human disease? They do so because people are vulnerable to many of the same or similar diseases as animals. Humans have 65 infectious diseases in common with dogs, 50 with cattle, 46 with sheep and goats, 42 with pigs, 35 with horses, and 26 with fowl. We have lived with and among these animals for thousands of years, so it is not surprising that we are susceptible to some of the same parasites, viruses, and bacteria as animals, including some that can be transmitted between animals and people such as rabies and malaria. Nor is it surprising that many chronic, noninfectious diseases such as epilepsy also afflict other species. The parallels between human and animal physiology and pathology were noted long ago, and the practice that we today call “animal research” has roots stretching back to ancient Egypt and Greece.
PHYSIOLOGY—All the functions of a living organism or any of its parts.
PATHOLOGY—The structural or functional manifestations of a disease.