Given that the virus has become far more severe and may very well spread to the States, how will it affect 2020?
We really don't know how severe its impact will be, especially in the US. Personally, I think most of its impact on the election will be in how it shapes other stories - what does or doesn't get coverage as a result of the 2019 coronavirus outbreak.
For example, if it gets a great deal of media attention, but doesn't really have much impact, it could be very good for Trump - distracting from his (apparent) dementia, and letting him claim he did a good job stopping it. On the flip side, if it is seen as a major problem for the US, that could end up exposing his inability and doing a great deal of damage.