You've breezed through the first few levels but now you're stuck. You keep waiting for the fungus to hit the last few island nations but before you know it, the damn scientists are dispatching the cure worldwide. The fungus pathogen is arguably the most difficult to beat but I'll tell you how, step by step.
But first, where to start? Most of the time I like to infect India first but for the fungus I prefer Madagascar, which will spread to either South Africa or Australia.
Wherever you choose to start, you'll probably want to switch the game to fast forward mode because you need to sit back and wait, silently as the fungus spreads. You can access fast forward mode by clicking the down arrow underneath the date in the upper right hand corner of the game screen.
Keep collecting DNA points by popping the red and orange bubbles.
The fungus level on Plague Inc. is so challenging because it is difficult for the virus to survive long distances (such as air travel or sea travel). Because of this you have to invest your DNA points in transmission. Depending on which country you start with, begin spending points on bird or cattle transmission (level 1) but save up points so you can upgrade water transmission to level two.
About now you should be getting notifications that your fungus is mutating causing new symptoms. In the past this was great news, a freebie but in fungus mode, symptoms are bad news. As soon as a new symptom developed you need to devolve it. To do this, click on the disease, then the symptoms tab, click on the brightly colored symbol and select devolve. As an added bonus you'll get two DNA points. We'll need those in a minute.
Because I'm impatient I like to upgrade both air and water transmission to level 2 and then upgrade to the bonus bioaerosol. While expensive, this will maximize the likelihood of the fungus surviving oversea travel. This is critical because it is the only way to infect the pesky areas like Greenland, Iceland and New Zealand. As long as you keep devolving symptoms, the fungus won't be discovered.
You need to keep your eye on how the fungus is spreading in countries. Again, Greenland and Iceland are notorious for having slow infection rates. If you start killing people too fast, all the infected people will die and can't infect new people.
Keep being patient, keep devolving symptoms and tracking your infection spread and saving up DNA points. I like to wait all the way until you receive the message "there are no more healthy people in the world". Once you get that message, unleash hell.
To do so, I prefer the symptom tree: coughing - pneumonia - pulmonary edema - pulmonary fibrosis - total organ failure - coma. For a full list of symptoms, their parent symptom and effects. Check out the Plague Inc. Symptom Encyclopedia.
With two lethal symptoms - total organ failure and coma, the world will be rid of humans faster than you can imagine.
Congratulations. You've beat the Plague Inc. fungus level.