> Decades later, Bailey concluded that the Isle of Wight disease was caused by a paralysis virus but viruses hadn't been invented at the time of the outbreak.

The quote was from Page, et al and is misleading, I suppose. On the other hand, what you say is not quite accurate either. Viruses were discovered prior to 1900. Sacbrood virus was found soon after:

> Since its first identification in the United States in 1913 (White, 1913), infection of SBV has been found on every continent where A. mellifera honey bees are present. 

Bailey associated chronic bee paralysis with the Isle of Wight disease:

> CBPV was identified as a cause of adult bee paralysis by Bailey et al. (1963) after long suspicion that the tracheal mite, Acarapis woodi, was the culprit of the paralysis.

In Bailey's words:

Severely affected colonies suddenly collapse,
leaving the queen with a few bees on neglected
combs. All these signs are those attributed to the so-called
'Isle of Wight disease', which is alleged to have
been prevalent in Britain early in the twentieth
century, but was of unresolved cause.

CPV occurs commonly throughout the world.
Infectivity tests indicate that the virus is endemic in
apparently healthy colonies in Britain but there is no
seasonality in incidence or in disease outbreaks.
Irregular factors that inhibit the normal activities of
bees probably increase transmission of the virus.

These factors include unseasonable weather, queenlessness,
crop failures and high colony densities, each
of which decreases the foraging activity of bees and
hence increases their bodily contact within colonies
during their usually active season. This may well
explain the close correlation between the decline in
the number of colonies in Britain and the decline in
the incidence of outbreaks of paralysis.

Sources:

HONEY BEE VIRUSES by Brenda V Ball, Leslie Bailey, in:
Encyclopedia of Virology, Volume 3, 1999

Honey Bee Viruses by Yan Ping Chen, Reinhold Siede, in:
Advances in Virus Research, Volume 70, 2007, Pages 33–80

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