These are core stats from Plague Inc. and Plague Inc: Evolved. The three main ones in Plague Inc. and Plague Inc: Evolved have the following descriptions, obtained via the manual:

  • Infectivity:
  • "How quickly the disease will spread!
  • Controls how fast the disease spreads both inside and between countries.
  • Very important!"
  • Severity:
  • "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"
  • Lethality:
  • "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."

Now, here is the main information about the core stats:


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 symptoms, but abilities and transmissions as well. Almost all symptoms can increase Infectivity somewhat. The Infectivity bar can (but will not do so normally when evolving Transmission) bring up the Severity or Lethality bar along with it, as some symptoms do have severe/lethal traits. However, governments can still 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.


Transmissions can help with spreading a plague to different countries. Bird IWater 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

Symptoms Edit

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.


Certain abilities help to spread a plague. Drug Resistance I 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.


  • 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 you get noticed on parasite after you evolve symbiosis.


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.


  • It is much like Pandemic's Visibility stat, except that bubbles don't exist in Pandemic.
  • 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.


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 disease 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 once all countries are infected and another highly lethal symptom can finish off the world fast.


  • 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.
  • Lethality is completely absent in the Santa's Little Helper scenario. All of the symptoms simply don't give any lethality.

See alsoEdit

Visibility is a measure of how visible your plague is to the world, and is a combination of severity and lethality. Although it isn't a stat in Plague Inc. and Plague Inc: Evolved, it replaces severity in Pandemic. If your plague hasn't been detected yet, then the more visible it is, the more likely it is to detected by the world. Visibility affects how much funding the world's governments will allot to development of a cure for your plague. It is typically recommended to keep Visibility as low as possible in the beginning of a disease, so as to not alert the governments of the world.

Happiness 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.

Trivia Edit

The Happiness stat bar replaces the normal Severity bar.