**1. Defining animal models**

 The use of animals as experimental models for human diseases is currently seen as an imperative in understanding the causes, biology, and prevention of diseases. Animal models over the years have been used extensively in biomedical field since the early 1980s [1]. Current understanding of these models tends to be a specific combination of an animal species, cell, tissue, organ, gene, or a challenge agent, and its directed route of exposure to produce and/or mimic a disease process or pathological condition in multiple important aspects approximating or corresponding to the human disease scenario or condition of interest. An important fact is that the models have to be reproducible.

It is obvious that laboratory animals play a crucial role in scientific research, discovery, and technological advances and in a substantive manner improve the lives of people and other useful animals. It may suffice to say that animals are used as models to study human biology and diseases and as test subjects for the development and testing of drugs, vaccines, and other biologicals (i.e., antibodies, hormones, etc.) to enhance and promote human health. This book, therefore, was written for medical practitioners, drug/therapeutic agent developers, biomedical scientists/bioengineers research students, bioethicists, behavioral scientists, and the general public who aspire to enrich their understanding of human diseases and development of effective therapeutics using animal models as clearly defined herein. Over the last century, almost all medical knowledge, treatment regimes, and medical device development have involved research using animals. Disease experimentation using animal models may be a deliberate design or an inevitable choice which possible due to the common descent of all organisms which even in the face of evolution many of them conserve their metabolic, developmental, and genetic material.

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