In today’s world, laboratory animals, particularly rodents, are housed in what are called biological barriers. Biological barriers, of which germ-free is the most strict, are designed to keep animals free from contact with microbes, and each level of barrier is defined by the microbes that are excluded and by the methods needed to create and preserve that particular barrier. Definitions and descriptions of the various levels of microbial containment are as follows:

Conventional animals are kept in open cages with no biological barrier and are not screened for microbes. Conventional housing is rare in modern animal facilities.