Interesting article [New Scientist, Scotsman] about how most house mice in Scotland, Wales Ireland have a characteristically Scandanavian genome, while mice in other parts of Britain have a genome associated with bronze age migrations of the first farmers, sugeesting that it wasonly in Viking times that the populations were sufficiently dense to sustain a mouse population with more intensive grain cultivation.
Likewise, prior to the black death we know the population had undergone a fairly rapid expansion, and hence could support the large rodent population in towns required as a reservoir for the plague bacterium. It has been hypothesised that the Black death was not a significant problem until it ended up infecting the large urban populations of Alexandria and Constantinople as part of the Plague of Justinian in 548. Large population, seaport, lots of rats.
There's also recently been a suggestion that the same sort of thing has happened with HIV in Africa - it had probably always been there but until significant urbanisation in the twentieth century, with accompanying population densities and greater opportunity for random sexual encounters.
So what does this mean for seventh century Britain. It's often argued that there was a plague event that preferentrially devastated the sub-Roman communities in the west of the island rather than the areas under Saxon control, providing an opportunity for further expansion westward by the Saxons. Does this mean that the plague was in the population and the outbreak was a result of higher population densities in the west capable of supporting a plague reservoir or does it simply mean that continued contact with the Byzantine east exposed the sub-Roman population to greater risk of infection as they had ports and the remnants of urban communities surrounding the ports to form an initial entry point for the plague?
If the former it suggests that the sub Roman successor states were capable of holding their own despite the loss of a lot of the prime agricultural territory, but to make a decision we need to know more about trade patterns, for example were the subroman populations also trading grain with northern France ?