These are the core stats from Plague Inc. and Plague Inc: Evolved, Lethality being represented by a skull symbol at the bottom of the screen, on the Death Toll Meter, and Infectivity being represented by a biohazard symbol at the bottom of the screen, on the Infection Count Meter. For diseases that don’t kill, such as Santa's Little Helper or Ultimate Board Games, the skull symbol is still a default. While it stays at zero in Santa's Little Helper, the dead people are replaced with “sales” in Ultimate Board Games. In Fake News, the skull symbol is entirely replaced with the healthy heart symbol for informed people.

"How quickly the disease will spread!
Controls how fast the disease spreads both inside and between countries..."
— Plague Inc. Manual, Infectivity

Infectivity, before, showing and after evolving Necrosis

Infectivity is a plague statistic that dictates how infectious the plague is. The more infectious a plague is, the more people it infects every day.

Infectivity can be increased from all of the trait types, not just transmissions, but symptoms and abilities as well. Almost all symptoms can increase infectivity somewhat. The infectivity stat can (but will not do so normally when evolving transmission) increase the severity and/or lethality stat, as some symptoms do have severe/lethal traits. However, governments still can detect your disease even without any severity or lethality. The chance of this happening increases as the difficulty increases. If a player wants to avoid their disease being detected, Coughing and Sneezing are probably their best bet for symptoms, while almost all transmissions and abilities do not raise anything but infectivity.

Players should note that although abilities do not appear to increase infectivity in the infectivity bar, they nevertheless increase the disease's infectivity in some countries.

Transmissions

Transmissions can help with spreading a plague to different countries. Bird I, Water and Air I are some examples that are useful to have in all country types.

Transmissions do not increase severity or lethality, unless you're playing the Neurax Worm DLC or Necroa Virus DLC.

Symptoms

Some symptoms can increase the plague's infectivity, but they will increase severity and, in the case of high-tier symptoms, increase lethality as well.

Abilities

Certain abilities help to spread a plague. Drug Resistance 1 and Environmental Hardening are good examples of infectivity increasing abilities. These abilities do not increase the bar itself.

Abilities do not typically increase severity or lethality, although the bioweapon has an exception: the Unlock Annihilate gene ability, which radically increases the severity and lethality of the plague.

Notes

  • It is displayed as a pink line on the Disease Graph, as it is a bar of the same color in the symptoms menu.
  • If you have exactly 1.5 infectivity points, you are undetectable. This is why your plague is noticed in the parasite mode after you evolve Symbiosis.

"A sign of how bad the disease is if you catch it!
It will slow down cure research and give you more DNA points
People will notice and care about a severe disease"
— Plague Inc. Manual, Severity

Severity, before, showing and after evolving Necrosis

Severity is the statistic represented by the yellow bar. The higher the severity of a disease, the more likely it will be detected by a government which may take action by starting research on a cure.

Once cure development has started, a higher severity makes it more likely that countries will invest more funding into cure research. A higher severity also makes the plague harder to cure; therefore, increments in severity also tend to also increase the length of time it takes to successfully develop a cure.

Higher severity also increases the amount of DNA points gained from red, bio-hazard bubbles and infected countries. Due to this, it is best to prioritise severity next after infectivity once the disease has been spotted, until lethality becomes important later.

Plague types can also increase or decrease severity. Parasite has a unique ability to reduce overall severity (Symbiosis 1 - 3). Bio-Weapon has unique abilities to raise (and lower) severity, and while most evolutions of the Neurax Worm tend to raise severity, there are symptoms that reduce the overall severity (Adrenergic Constriction and Anxiolytic Infusion). The mutations from Viruses can lead to massive increases in both severity and lethality if left unchecked.

Trivia

  • It is displayed as a yellow line on the Disease Graph, much like it is displayed as a yellow bar on the Symptoms menu.
  • Severity also increases the amount of DNA Points you get from popping red bubbles.
  • Severity exists in the Santa's Little Helper scenario, with the same function, but is instead called Happiness.

"How easily the disease can kill someone!
It can slow/stop cure research and give you more DNA points.
People will really notice and care about a disease that kills."
— Plague Inc. Manual, Lethality

Lethality, before, showing and after evolving Necrosis

Lethality is a plague statistic that dictates how deadly the plague is. The more deadly a plague is, the more effort people will put into developing a cure for it.

At the beginning of a game, lethality is not in any disease unless a deadly symptom like Total Organ Failure, Necrosis, Hemorrhagic Shock or Coma is added. Bio-Weapons will also slowly begin to generate toxins, gradually increasing their lethality unless suppressive abilities are evolved.

Unless special plagues or Scenarios are played, killing off all humans is the only way to win the game.

Once lethality starts to take effect in gameplay, countries seem to become more likely to close their airports, seaports and land borders. Overall, lethality is best to be prioritised last in gameplay once all countries are infected. Corpse transmission symptoms and abilities can easily infect the last of the healthy people when all countries are infected, and another highly lethal symptom can finish off the world fast.

Trivia

  • It is similar to the Deadliness statistic in the Pandemic flash game.
  • When increasing lethality, the models that represent the player's plague will begin to animate more in Plague Inc.: Evolved.
  • Lethality is completely absent in the Santa's Little Helper scenario. All of the symptoms simply don't give any lethality.

Scenarios use core stat bars the same way, but with different names. Severity is replaced by Happiness, which is a stat bar exclusive to the Santa's Little Helper scenario. The happiness bar will tell the player how happy people infected with the Neurax Worm are, and replaces severity as the middle bar. It has a yellow color. Certain things will bring up happiness, including the symptom combos On The Naughty List and Ultimate Christmas, and most of the symptoms, save ones from the Sloth and Indulgence trees, the only traits which decrease happiness in the scenario. Transmissions do not increase happiness, however, but some abilities, like Red-Nosed Guide and Red-Suited Pilot do.

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