What level does the virus research facility need to be in order to occupy a level 5 land plot?

You can check the stats on each plot by tapping it and selecting View. Look for the infection intensity. It will also show your current immunity level. Each virus research level will add 100 points to your immunity. Once your immunity is greater than the infection intensity you can safely occupy the plot. You can also try to occupy the plot before then but you will suffer a lot of troop losses and its likely you will fail.

Level 5 plots seem to have an intensity of 1000, so you will need 10 levels of virus research to tackle these safely. You can build more than one virus research center though by using your speciality points in the Blue section.