Newfoundland is an island and it was, at best, extremely sparsely populated when the Vikings arrived. According to Wikipedia [1] the estimated local population when Europeans arrived in 1497 was 700 (that's on an area only slightly smaller than England). So I'm thinking that contacts were very limited and the potential for any disease to spread minimal.

[1] https://en.wikipedia.org/wiki/Newfoundland_(island)#First_in...