In a discussion of economic threshold levels on the Irish Beekeeping
List, Norman Carreck, wrote:

The important thing to bear in mind is that some viruses are more
virulent than others, and therefore, logically, there would be a
different threshold for each virus. It may seem counter intuitive, but
experimental work has demonstrated that a greater number of mites are
required to cause a outbreak of the more virulent viruses such as slow
paralysis virus (SPV) or acute paralysis virus (APV), than with viruses
that are not rapidly fatal such as deformed wing virus (DWV). If you
think about it, though, all becomes clear, because a virus which kills
the bee in a few days stands a much lower chance of being passed on to
another bee. The figure of 2500 mites, on which the Varroa Calculator
was based, was based on data from about 70 colonies studied by both
Rothamsted and CSL over a number of years. Some of these colonies, in
the early 1990s had no viruses present, and survived with huge numbers
of mites present (I think 24.000 was the record), but the remainder had
one or more viruses present and succumbed at a much lower level, so
2,500 was chosen as a realistic level for practical use.

The pattern of virus incidence undoubtedly changes over time. In the
early stages of an outbreak, when mite numbers are large, the more
virulent viruses predominate: APV in continental Europe, SPV in the UK,
and now Kashmir bee virus (KBV) in New Zealand. In the UK, because most
beekeepers control mites effectively, SPV has become rarely a problem,
but DWV is now universally present in colonies.

and to my point about viruses multiplying in mites:

Although this has been suggested a number of times, and has appeared in
a number of scientific papers, I have as yet seen no convincing evidence
that viruses multiply within mites. All the evidence suggests that mites
are purely passive vectors.

and to my point about wounding being an issue connected with virus
multiplication:

There is no doubt that DWV is now much more common than it was pre
Varroa, and more importantly, the mite transmits to a life stage (i.e.
brood) in which it was not present previously. I don't think there is
any particular evidence that the physical wounding has a harmful effect,
but there is growing evidence that mite feeding does actually interfere
with the bee's immune system allowing viruses which would not normally
multiply, to do so.

I hope this makes a useful contribution. The longer a colony can keep
off dwv, (i.e. not show the signs) the more likely it will become one of
my breeders.
--
James Kilty

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