Another interesting article from the UK's Daily mail newspaper about the concern Russian scientists have about old, dormant viruses "waking up" and starting to spread again. 

https://www.dailymail.co.uk/news/article-10307853/Melting-permafrost-release-potentially-deadly-ancient-viruses-Russia-warns.html - https://www.dailymail.co.uk/news/article-10307853/Melting-permafrost-release-potentially-deadly-ancient-viruses-Russia-warns.html

I was reading an article that was talking about viruses vanishing in history. They gave an example of viking bodies that were found and analyzed. Apparently they found that the bodies had a totally different strain of smallpox than the current strain we have/had now and that it went extinct and vanished some 1400 years ago.

I don't discount the possibility that a virus from the past might well someday find it's way back to the present. 

I've read an amazing book twice about the Black Death. The authors were well qualified scientists who did a forensic analysis of the Black Death and concluded there were a few different strains but that the most deadly had an incubation period of up to 37 days. What was particularly interesting was that they showed, through examination of the historical records, that this disease would disappear for years and then suddenly reappear, seemingly out of nowhere, which means that it must exist in an animal reservior. The question then ended with was when will it reappear? They could find no reason to suggest it had disappeared for good.

At my earliest opportunity I'll post the title and authors. I keep meaning to buy my own copy as it's such a good book.