Joe Biden is 77 years old; from actuarial tables 77-year-old men have a roughly 4% probability of dying this year, and hence a roughly 1% chance of not surviving until the Democratic National Convention. Additionally, there is some chance that he contracts coronavirus, which has a high mortality rate for people in his age range.
If the Democratic candidate does die, then the Democratic party will pick a replacement to nominate - this process would likely be very similar to a contested convention, with delegates and superdelegates not bound by the results of the previous primaries and caucuses. As the second-place candidate, Sanders would be a natural choice for the nomination, but need not necessarily be the nominee.
If Biden has the majority of pledged delegates, but does not survive until the nominee is chosen, will Sanders become the nominee?
The nominee is likely to be chosen at the 2020 Democratic National Convention, but positive resolution does not require the nominee to be chosen at the Convention (in case it's cancelled and another procedure is adopted).