The Black Death

[Prince Cobra]

Yes, it can and will transer from human to human. There was an outbreak in India a few years ago. It got pretty scary for a while (had an epidemic in the making) until enough penicillin and vaccines got sent in.
The bubonic plague can indeed infect from person to person, you would just need any bodily fluid. And it wasn't as if people were exactly clean or even stayed clear of the sick (besides the lepers), so person to person transfers does not seem too far out.
Anyway, the rat theory is getting a lot of flak these days. Quite simply the rats didn't and don't move about very much, and their fleas while able to spread the plague would die after the first bite on humans, so they couldn't really jump back to new rats or other humans.

If it truly had been the rats then we would have seen a much more different pattern than the rather linear advance. We would have seen outbreaks quite soon after the first in Italy in the big Med. ports, creeping around to the Atlantic. Possibly reaching England a few months after the first outbreak. Now that didn't happen... Instead the plague slowly pushed upwards from the landside.
I do not hope that anybody would argue that rats were relatively rare on ships. Or that new rats were transported aboard whenever (mostly) new cargo got lifted on.

Kraxis, IMHO the explanation of the Black death is far more complicated. On the one hand people can transmit the disease among themselves but in the other we have exhausted human organisms ( not because of the climate, but mainly because the Europe was rather overcrowded at that time ) and the fleas . All this thing made the Black death extremely lethal.
About the ports: as far as I know Genoese sailors brang the disease. Maybe the reason why the disease came later in England is that there was any kind of quarantine.
About the fleas: I have found some interesting facts - yes, the disease was lethal to the flea but it make the flea starve and forced it to bite far more frequently.

P.S. The disease was unavoidable neither the good hygiene ( for ex. Byzantines also suffered although they took care of their hygiene) nor the quarantine could stop it. They could only reduce it.