How to successfully infect all people in plague inc

I'm currently struggling to infect all people in plague inc. My disease kills too much people, and all the infected die without infecting more people. So, I always lose as a result. Is there a strategy to play this game? How can you evolve the disease specifically so it's not too deadly, but also can infect the whole world?

TL;DR

Increase Infectivity as much as possible.

Until all humans are infected, keep Severity as low as possible (do not increase it, if possible), and do not increase Lethality.

Don't evolve Total Organ Failure until all humans are infected, devolve it if necessary.

This will depend on the type of disease, but in general it boils down to the following:

1. In the beginning, focus on Transmission and Abilities. Avoid increasing Severity until after you've been detected, or after you've infected every single country. Even a single point in Severity will eventually lead to detection (in Mega Brutal difficulty, you can still be detected with no severity), which will make it harder to infect people. The higher your Severity, the faster the Cure Research will be, so try and keep it low even after you're detected.

2. Don't increase Lethality until after you've infected every single country. If your disease mutates a lethal Symptom before then, devolve it. This does not apply to Viruses, as they mutate too quickly and cost DNA to devolve. The only exception to that rule is Total Organ Failure (see 4.), make sure you always have enough DNA to devolve it as soon as it mutates.

3. If you can, try and prioritize infecting islands with no infected. Your disease can still spread to neighboring countries even when borders are locked down and animals are being killed. However, your disease can never cross over ship or airplane once services are shut down. As a result, if you are detected, you'll want to maximize Air and Water Transmissions as soon as possible to keep infecting planes and ships despite filters.

4. Absolutely do not ever evolve Total Organ Failure until after you have infected every single human. If your disease mutated it, devolve it immediately. Even if you think there aren't many healthy humans left, Total Organ Failure will kill the sick ones fast enough for the remaining humans to stay healthy and survive.

Of course, each disease type will have slightly-to-heavily different strategies:

• Bacteria: No special strategy.
• Virus: Never evolve a Symptom, the Virus will mutate them itself. Don't bother with Severity or Lethality, just focus on getting rid of Total Organ Failure. Once everyone's infected, evolve Total Organ Failure, or sit back and enjoy.
• Fungus: This disease type spreads very slowly, so keep Severity at zero until after you've infected everyone. You can evolve Spore Burst many times to help infect new countries, so Patho-Stasis may help keeping DNA costs low. Spore Eruption is best used when there's 10 healthy countries left. Once every human is infected, evolve only the minimum needed to get Total Organ Failure. If you don't have enough DNA for that, at least try to gain as much Lethality as possible.
• Parasite: You start with some severity, so maximize Symbiosis as soon as possible to reduce your starting Severity. This also increases Infectivity, which is always good.
• Prion: No special strategy.
• Bio-Weapon: Focus on delaying and reducing Lethality. Reset Lethality only if it's reached high enough levels. Once all humans are infected, "Unlock Annihilate gene" and enjoy.
• Nano Virus: Since you're detected from the start, focus on slowing Cure Research. Since every country is giving maximum priority to Cure Research, there's no need to worry about increasing Severity.