Gnotobiotics is the scientific study of animals and other organisms that are raised in germ-free environments, or ones that contain only specific germs.
These animals are first removed from the womb (usually by cesarean section and then placed in sterilized cages called isolators. Scientists are then able to use these animals to determine how specific pathogens such as bacteria, viruses, and fungi, each affect the body.
The root of the word is a combination of "gnotos" meaning "well known" and "biota" meaning the combined flora and fauna of a region. This is because the scientists know what germs the animal being studied is exposed to, so most irrelevant variables are eliminated.
There is a gnotobiotics association at http://www.gnotobiotics.org/