No guys, WienerBlut has it perfectly here. Most diseases like smallpox have a very small incubation period, while on the other hand having a very high mortality rate, especially in virgin field epidemics. Even if, and its a big if, some Norseman with smallpox or etc., lived long enough to bring it to Vinland, and transmit it to the natives, the die-off would be so severe it would never spread beyond the immediate area.
The reason the death toll in Mesoamerica was so high was partially because the Spaniards stuck around continually reinfecting the native population with successive waves of disease.
It seems for me that an incubation period of around 12 days is a long incubation period.
If you have 10 to 14 days between contraction and the first obvious symptoms of the disease, even on foot, somebody healthy even in difficult terrain can spread the disease in a big territory. And the infected person is contagious until the last smallpox scab falls off.
In OTL, the english and french colonies on the northern atlantic coast don't find many natives as if the smallpox and others european diseases had already killed them.
At the end of the 18th century, the disease spread among the Great Plains indians with little contact with the europeans settlers and in a low density environment and was a major reason for the rise of some tribes and the decline of others...
Even if it is Wikipedia, this article give good informations of the rate of mortality of the smallpox even in natives culture less modern and less urbanized than the Aztecs or the Incans empires.
http://en.wikipedia.org/wiki/History_of_smallpox
If the Vinlanders will be able to establish even limited contacts with the Natives in what is OTL, Labrador, Quebec, Nova Scotia or even New England, the disease will spread quickly and even more quickly if the Vinlanders spread the disease by boat, each coastal tribe they will contacted and traded with will become a new point of spreading the disease in the hinterland...