Map visualization; relative abundance and directional change
A good starting point for the data visualization would be a relative abundance (fever cases per all entries per zip code) heat map. Color ramp either from neutral grey to deep red OR green to yellow to red. To normalize the color ramp maximum, some sort of scaling is needed; can be either maximums from the country/continent/zoom_area - if easier in the beginning, can be set manually in the first version.
Also, what could be interesting and helpful, would be a temporal direction of change map, so having the last day's abundance data compared to a certain number of earlier days average, like the last three days before the last day. This would be visualized with blue if the ratio is decreasing, red if increasing and neutral grey if staying about the same; the level of buffer, so what is large enough change in ratio to be visualized as such, has to be decided; difference of 0,1 to 0,2 % would be probably in the right scale.
A map layer with counts of all individuals and individuals with fever per zip_code/country/zoom_area is a good layer also, for people to evaluate the reliability of the data.
I'd also try out the different map layers scrollable on the page (I know this might be annoying with moving the map and zooming, so has to optimized) instead of having a dropdown menu. So having all the different layers as separate maps. It's easier for people to use and compare when quickly looking through the data.
Related to this: