I spent a little time trying to determine what happens if Trump gets the virus and passes away . . . . . after coastal America concludes their parades and celebrations, I mean.
Obviously Pence steps in at the top. However, worrying news:
As best I’m able to determine, Pence would nominate a new VP. That person would need to be confirmed by BOTH the Senate (no worries there) AND the House (major trouble!).
Absent both those confirmations, 80-year-old Nancy Pelosi would be next in line of succession. 
In normal times, without a deadly virus out there, all that would be wildly far fetched. But given our situation now, loss of both Trump and Pence is not impossible. And the thought Pelosi would roadblock confirmation of Pense’s VP choice, to her own advantage, is not far-fetched in the least.