Difficulty is a feature in Plague Inc. and Plague Inc: Evolved that alters gameplay according to difficulty. If you beat a plague on any difficulty, all the previous difficulties for any scenario or plague type will be registered as beaten as well, indicated by biohazards. Some features are locked until you beat the game in the required difficulty (and above). For example, you can beat Bacteria on Brutal instead of Normal to unlock Virus.
Casual is the 1st and "easy" difficulty, and is for those wanting a quick or easy game. It is the minimum to unlock a random gene of the Genetic Code respective to its plague type. In it, people are extremely lax. People do not wash their hands, doctors don't work, and sick people are even given hugs "For new players or those wanting a quick game"
- No one washes their hands — Fast infection rates.
- Research doctors don't work — Slow cure research speed.
- Sick people given hugs — Infectivity increased once spotted.
Normal is the 2nd and default difficulty, and is the minimum for unlocking the next standard plague type. In this difficulty, more people wash their hands, as well as doctors contributing to research. "For experienced genetic strategists"
- 67.3% of people wash hands — Adequate infection rates.
- Doctors work 3 days a week — Adequate cure research speed.
- Sick people ignored — No change in Infectivity once spotted.
Brutal is the 3rd and "hard" difficulty, and was the hardest before Mega Brutal was introduced. Brutal introduces obstacles such as excessive hand washing, very determined doctors, and higher disease paranoia. Winning with all standard plague types on Brutal unlocks the Neurax Worm, and winning with the Neurax Worm on Brutal unlocks Necroa Virus and so on. "For strategic geniuses living in concrete bunkers"
- Compulsive hand washing — Slow infection rates.
- Doctors never go home — Fast cure research speed.
- Sick people locked in prison — When disease is spotted, infection rates are greatly lowered.
Mega Brutal is the 4th and "very hard" difficulty. Introduced in Plague Inc: Evolved and later added in the Mutation 1.8 update for the mobile version Plague Inc., this difficulty can have slightly longer playtime, and doctors can and will discover a disease without symptoms. Doctors will then stop at nothing to cure diseases. "For players who thought Brutal was Casual!"
- Genetic drift impacts evolution - Symptom costs become more expensive with more people infected.
- Doctors invest in research — Very fast cure research speed.
- Random medical check ups — Plague will randomly be detected through "Anomalous blood test results". It will also be detected automatically once any country is fully infected.
Casual is the 1st and "easy" difficulty, and is for those wanting a quick or easy game. In it, people listen to experts, aware of fake news, and countries cooperate to eradicate the disease. "For new players or those wanting a quick game"
- Full international co-operation — Fast cure development and more authority.
- Clapping scares disease away — Low infection rates.
- People love wearing masks — Low transmission rates.
Normal is the 2nd and default difficulty. "For experienced genetic strategists"
- Politicians vaguely competent — Adequate authority.
- Experts mostly listened to — Less non-compliance.
- Healthcare systems unprepared — Fatality increased.
Brutal is the 3rd and "hard" difficulty. Brutal introduces obstacles such as fake news, politicization of science, and even arresting experts who report on the disease. "For strategic geniuses living in concrete bunkers"
- Sick people inject bleach — Fake news and higher fatality rate.
- Leaders ignore science — Authority depletes rapidly.
- Doctors arrested for reporting disease — Experts not listened to, non-compliance increases.
Mega Brutal is the 4th and "very hard" difficulty with slightly longer playtime. This features careless, ignorant and unintelligent citizens, politicians and doctors alike. "For players who thought Brutal was Casual!"
- Sick people given hugs — Very fast transmission.
- Everyone believes fake news - High fatality rate and high non-compliance.
- Doctors play Plague Inc. all day — Very slow cure research and high fatality
- Completing a game in the Casual difficulty will not unlock a new plague unlike the other difficulties, but it will unlock new random genes (or an advisor if you are playing Cure Mode). This way, the same game setting can be played over and over again, and you will easily get all the genes without much trouble.
- Plague Inc: Evolved have "Pity Mode" in the settings, allowing the next plague type to be unlocked by beating the previous one in Casual.
- In order to access Mega-Brutal in Plague Inc., tap the biohazard icon below the Brutal difficulty, which caused Brutal to be replaced by Mega-Brutal. It also changes the icon into black-in-gold, and it can be activated/deactivated at will.
- In Plague Inc: Evolved, it's available separately.
- On Android, you must buy the premium version to access the Mega-Brutal difficulty.
- The description for Brutal Difficulty in Cure Mode contains two references to COVID-19, specifically two scenarios that hindered attempts to act against the spread and treatment of the disease, one of which where the President of the United States at the time suggested injecting bleach to cure COVID-19. Another, Dr. Li Wenliang attempted to warn about the virus and was arrested by the Chinese Government, and later died due to COVID-19 infection.