You do make a good point, unfortunately, on both. But then again, this also depends and goes on the assumption that unlike the Will Smith film, "I am legend.", when his beloved German Shepard "Sam" was bitten by another "infected" dog and she too contracted the biological agent and "turned". As I said, assuming dogs (God willing) would be immune to what ever 'bio-fubar' humanity lets loose.
Just a side note; I do believe "Legend" was a rewrite of the 70's Charlton Heston film, entitled IIRC, "The Omega Man" But I really enjoyed both.