The fungus level is the only one I always play passively in. Fungi are slow as molasses, so you can't risk being discovered before you at least have a foot-hold in every single country. Devolve any random mutations you get, besides coughing, which increases infectivity, and from what I've observed, doesn't get you caught. Just focus on upgrading Land/Water transmission to 2, adding a point into Cold for Greenland, Blood for Africa, and Drug Resistance for the developed countries.
Once you have those set, you basically sit on your hands and twiddle your thumbs for four years until you've infected every country. Then you can set the kill-switch to on, and get all the symptoms you like, as well as Genetic Hardening and Restructuring as needed. It's not that difficult to win as a Fungus, just boring. Oh, and if you haven't already infected everyone, be sure to pick up Necrosis to make sure it gets around.