'Virus' is Latin, sure, but it's fourth declension, not second (and certainly not "declination"!), hence the plural (and genitive, if that means anything to you) in Latin is 'virus', with a long 'u' instead of a short 'u' as in the singular. I won't go into whether or not that should be the correct plural when used in English.

And FYI, the 'pus' in 'octopus' is actually of Greek origin, so really the plural ought to be 'octopodes', as AU #64 said, though the reasoning isn't quite right. Again though, that's according to the language it was borrowed from, and doesn't necessarily make it right for English.

--
The problem with defending the purity of the English language is that the English language is as pure as a crib-house whore. It not only borrows words from other languages; it has on occasion chased other languages down dark alley-ways, clubbed them unconscious and rifled their pockets for new vocabulary.
  - James Nicoll, 1846 - 1918

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