The Fungus is a plague that introduces Unique Traits. It is unlocked by completing the Virus on Normal (or, in Pity Mode, Casual). The guide for the Virus is provided below.
https://steamproxy.net/sharedfiles/filedetails/?id=2351408230

The Fungus plague has 3 unique abilities that will help it spread faster internationally in spite of its poor cross-country transmission (which is 1/10000th the norm), however, it has a considerable bonus in humid climates. Due to how gimmicky the fungus is, it is extremely long and drawn out, but not difficult. Arguably, this plague is the most luck-based as you may not be able to infect every country before The Cure due to the near-perfect True RNG (as opposed to the common pseudo-RNG).

Unique abilities will be explained as they are used.

Genes aren't necessary for the fungus, but will substantially speed up its long, tedious process.

Use Metabolic Jump or Cytochrome Surge, which increases DNA from Red and Orange bubbles, respectively. Alternatively, use Metabolic Hijack to pop these bubbles automatically.

It is recommended to use Genetic Mimic here, as it slows the cure (just in case).

The best choice here is the Suppression gene, which increases the astronomically low (more specifically, 0.0001%) chance of the fungus crossing a border.

While Extremophile is preferred, Xerophile can be used to make the infectivity in arid and humid climates more equal.

Ionized Helix gives you more DNA when you devolve, and it's the only one worth using.

The Fungus will provide 2 DNA for devolving a symptom. Note that the Fungus is RNG based and takes a very long time, so be on the lookout.
DEVOLVE ALL SYMPTOMS THAT MUTATE.

Begin in India (or, more specifically, the countries of Bangladesh, India, and Sri Lanka (and, if on iOS, Nepal)). India is a rural, hot, humid country and a good spot to start as it has a very high population (1,457,015,015, to be exact), an airport, a port, and borders 3 countries (which doesn't matter due to the astronomically low chance of spreading past a border).

Save up for 66 DNA.
Under Transmissions, get Air I, Air II, Water I, Water II, and Extreme Bioaerosol.

Save up for 33 DNA.
Under Abilities, get Drug Resistance I, Cold Resistance I, and Cold Resistance II.

Save up for 75 DNA.
Under Abilities, get Heat Resistance I, Heat Resistance II, and Environmental Hardening.

Now that you're here, you likely have a few more countries left. If there are still uninfected countries, use the special abilities to infect them. These are 12 Spore Bursts, which will infect 1 random country that hasn't been infected, 2 Spore Eruptions, which will infect 5 countries, and Spore Hardening, which will infect every country. Only use as many as you need, as these can't be devolved.

Check the World Menu and make sure EVERY COUNTRY is fully infected (Red, not Pink). This is in case everyone is infected and you haven't gotten the notifier for it.

If no country in the world map has a PINK or BLUE border, and/or you have gotten the message "THERE ARE NO HEALTHY PEOPLE LEFT IN THE WORLD," move on to part 3.

Obtain the message "THERE ARE NO HEALTHY PEOPLE LEFT IN THE WORLD" and/or have no pink or blue countries in the World menu.
Make sure that all symptoms were devolved. Under Abilities, DEVOLVE Drug Resistance I, Cold Resistance I, Cold Resistance II, Heat Resistance I, Heat Resistance II, and Environmental Hardening. Under Transmissions, DEVOLVE Air I, Air II, Water I, Water II, and Extreme Bioaerosol. THIS IS EXTREMELY IMPORTANT.

Make sure you have at least 50 DNA.
Under Symptoms, evolve Coughing, Sneezing, Immune Suppression, Total Organ Failure, and any other lethal symptoms. Allow mutations.

You win!