Why do we use animal for medical research instead of useing humans for medical research?

i really need information for this for my ag class so please answer it as soon as possible.

Why do we use animal for medical research instead of useing humans for medical research?
I often thought the same thing like why not allow death row inmates to offer their services to the public by being used in medical experiments. The degree of severoity of the medical condition would have a direct bearing on the type of crime commited.
Reply:Actually, they start out with animals and when the animal survives, they try it on humans. It's bad enough that their are cases where human's are tested and the phrase is considered guinni pigs as silently said for research, and their very close to believing that whatever their using will work of humans. "Just pray you'll never have to be one of those"... Dr Checko... (Nickname)
Reply:because people are not willing to be used as research unless they are deceased...Animals are forced....