The significance between the difference of the two has now been elucidated with study of the rabbit-lethal myxoma virus. Many species of rabbit cells in culture (without the presence of any antiviral defenses that would normally be in a host) can be infected by the myxoma virus, causing infection and cell death. However, inoculation of the myxoma virus in many species of rabbit shows that only one species of rabbit is affected, the rest being completely unharmed by the virus (lack of even viral shedding). This has been determined to be a result of the myxoma virus's inability to suppress other species' interferon expression, and hence resulting in the interferon in turn suppressing the myxoma virus.
This is a result of the positive susceptibility of many species of rabbit's cells, but negative permissibility of all but one of the rabbit species' cells.