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I'll expand a bit on this. Although the lab animals tend to be healthier, it's largely because genetic disorders are bred out of many of them. However, many are very susceptible to infections and contagious pathogens. Their immune systems are often compromised to the point where they have to be kept in very clean, almost sterile conditions. The reason has to do with how immune system genetics work. It's different from the normal genetics of other traits. It's actually very complex, but basically through a process of recombination, gene segments combine randomly to create enormous numbers (millions) of new genes, each capable of fighting a different problem. The more diverse it is, the more problems it can fight. This system depends on diversity of the parent-stock immune system. Inbreeding eventually causes a loss of diversity and can only be recovered with outcrosses to stock that contain different gene segments. Linebreeding was developed to combat this problem without losing too many of the positive traits of the group. Linebreeding allows for a more diverse immune system by keeping several groups of related individuals that are systematically crossed to each other. In other words, it's a method of controlled inbreeding.