Smallpox and measles (two candidates for highest body count in the New World) weren’t established in the Viking part of Europe at the time they voyaged West. Also, Viking culture wasn’t highly urbanized, and a certain level of urbanization is needed to sustain a pool of infectious diseases inbetween epidemics. The rise of plagues like the various poxes goes hand-in-hand with the rise in urbanization, and the people coming to America after 1492 were from those highly urban cultures (so often were asymptomatic carriers or recovered victims), unlike the Vikings.