This article is from the Computer viruses FAQ, by David Harley D.Harley@icrf.icnet.uk, George Wenzel firstname.lastname@example.org and Bruce Burrell email@example.com with numerous contributions by others.
The Latin root of virus has no commonly used plural form. Since the
use of the word virus is borrowed from biology, you might like to conform
to the usage normally favoured by biologists, doctors etc., which is
viruses. However, a number of people favour the terms virii/viri,
either to avoid confusion with the biological phenomenon (but what's
the point of distinguishing in the plural but not in the singular?),
or to avoid being mistaken for anti-virus researchers.....
Bottom line, 'viruses' is the correct English plural for the singular
'virus'. Viri, virii, and so on are all slang.