Scientists use animals to learn more about health problems that affect both humans and animals, and to assure the safety of new medical treatments. Some of these problems involve processes that can only be studied in a living organism. Scientists study animals when there is no alternative and it is impractical or unethical to study humans.
Animals are good research subjects for a variety of reasons. They are biologically similar to humans and susceptible to many of the same health problems. Also, they have short life-cycles so they can easily be studied throughout their whole life-span or across several generations. In addition, scientists can control the environment around the animal (diet, temperature, lighting, etc.), which would be difficult to do with people. However, the most important reason why animals are used is that it would be wrong to deliberately expose human beings to health risks in order to observe the course of a disease.
Animals are needed in research to develop drugs and medical procedures to treat diseases. Scientists may discover such drugs and procedures using research methods that do not involve animals. If the new therapy seems promising, it is then tested in animals to see whether it seems to be safe and effective. If the results of the animal studies are favorable, human volunteers are asked to take part in a clinical trial. The animal studies are done first to give medical researchers a better idea of what benefits and complications they are likely to see in humans.