Why is my Fungus not spreading/killing?

You need to go to Abilities and evolve things like Cold Resistance, Heat Resistance, and Drug Resistance. Otherwise, your disease will only progress very, very slowly.

If you start in a hot country, you pretty much need Cold Resistance for Greenland and Iceland.

I've seen this happen in two places:

USA / Japan: Solution: evolve drug immunity, on Brutal you need to go to level 2.

Greenland: Evolve Cold immunity, on Brutal, you need level 2.

However, with Fungus, I haven't seen the last ability really work well - where "spores are constantly released". I wonder if it's related.