I find that pokerus seems to spread far faster if the carrier is a higher level than the recievers, and the greater the level difference, the faster (using evidence from my Diamond, in which... [long, so I'll stick it below])
Unless it's the daughter of my mukkuhawk, who has stubbornly not caught it at all.
Possibly it's to do with family lines? (my mukkuhawk has been antibodied since long before he had his daughters)
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At some point in time while I was training to get Otoojio, my starter, to evolve (into empoleon), he caught the Pokerus. This passed to Denkou (shinx>rentoraa), who passed it to the pokemon that I now keep in my box (a level 5 zubat and level 13 onix).
Through them, Umino (magikarp>gyarados) and Hishoga (ponyta>rapidash) caught it.
It took a while.
So my observation goes: the higher the level of the pokemon that has it, the faster the weaker pokemon catch it.
Den' had it as soon as I checked her stats, directly after capture (Otoojio was level 36 to her 3/4). I had to infect the rest of my team (whom I caught after Otoojio and Den's 3-day spreading period was up) by putting my box pokemon (the above mentioned [5]zubat and [13]onix (who had caught it from Den'/ Otoojio, meaning, immediately)) in my party. It took considerably more time to transfer the virus, and Umino, who was the weaker of the two [level seven to Hishoga's thirteen], caught it first)
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So possibly the not-transferring issue a) is the levels, b) is something to do with family lines (genetic immunity so the antibodies never show up?)(I honestly doubt this one, but eh. It's interesting), or c) something strange is happening.
I don't know, because my White is a Shiny game, not a PokeRus game, so I can't check to figure it out in that Gen.
