The standard way to simulate aggression in Plague Inc's normal plagues is to raise the lethality, but that's not as fun as taking over the world. Let's try something better.

It's possible to create entirely new mechanics in Plague Inc using the tricks outlined in the Advanced Text Editing Guide, but that's tricky to learn and inconvenient to use. So here's a file for a highly-demanded mechanic, with most of the hard work done for you!

This is a simple downloadable file that uses advanced event editing to simulate combat. It's a big file, but you can ignore most of it - all the magic happens in the _Stats_Update event, which lets you control the attack, defense, and behaviors of the infected based on evolved traits or custom events.

Get the file here: https://indigofenix.com/files/plagueinc/monster_combat/events.txt
(Right click and press Save As)

First, you'll need to create a new scenario in the Scenario Creator. The Monster Plague Engine works with any plague type except for the Simian Flu (but see caveats for special plagues below). You will need to have the Events Lab enabled.

Once you create your scenario, you'll need to find the folder where its files are kept. On Windows, this will be:

Documents/Users/(Your Name)/AppData/Local/Ndemic Creations/Plague Inc. Evolved/Scenario Creator/(Scenario Name)

(AppData is sometimes hidden. You will need to show hidden files[www.technipages.com] to access it)

Download the above file and drop it into this folder, overwriting the existing events.txt file if it exists. Then open up the scenario in the Scenario Creator (if it's already open, you will need to reload it.) Open the Events Lab and you should see a _Stats_Update event and a whole mess of z_country_update events. If you do, you've done it correctly and can move on to the next step!

Ignore all the z_country_update events. This is where the actual combat processing takes place; you don't need to touch these.

(There are 58 copies of the same event, one for each country - annoyingly, this is the only way to make an event run once in each country every day. Ndemic, if you're reading this, could you make a "run event once a day in each country" option? It would make these things so much easier.)

All you need to pay attention to is the _Stats_Update event. This event will run once a day and will update all the relevant stats. The simplest way to do this is to start with your base stats, and then add a bunch of "If" statements that add or subtract these stats if a particular trait is evolved, or if any other conditions are met. This event will run every day, and once it's done the stats will be at the level they are supposed to be.

As an example, I have added one If statement that triggers if the "Aggression" trait (of the Neurax Worm) is evolved. Since the stats use Ape variables, you will not be able to select them normally (without text editing), but you can copy this If statement and change the condition and the variables inside.

As for what the stats actually do:

• Ape Strength: Likelihood of infected to kill the uninfected each day. This is calculated as a percentage of the total infected in a country.
  
• Cross-Species Infectivity: Likelihood of infected to forcefully infect the uninfected each day. This operates on top of the normal Infectivity rate and ignores all government actions that impact the infectivity in a country.
  
• Ape Lethality: The speed that humans will start arming themselves against the infected, impacting both attack and defense. You'll probably want to start with this at 0 and then set it once the "begin combat" trait is triggered, but this is up to you. See more details below.
  
• Ape Survival: This improves the defensive ability of the infected, letting them survive attacks better.
  
• Ape Horde Speed: Reduces the impact of human preparation when calculating results of infected attacks on humans. Basically armor penetration or surprise attacks.

As countries grow more worried about the plague (due to either severity or deaths), they will start arming themselves at a speed based on the "Ape Lethality" value. A country's combat ability will increase this amount each day, until it reaches:

(Global Awareness + Country Awareness) / 2

And then this value is multiplied by Country Public Order to calculate the actual combat strength at a given moment.

Each day, a number of uninfected will be killed. This number is:
Country Infected Population * Attack Power / Country Combat Strength / 100
Each day, a number of uninfected will be infected. This number is:
Country Infected Population * Infection Power / Country Combat Strength / 100

After this, a number of infected will be killed. This number is:
Country Infected Population * Country Combat Strength / Defense Power / 100

Important to note: Human combat ability is not based on the number of healthy people in the country. It is based on the Country Public Order. This means that even if the country is completely infected, infected will keep dying until the country descends into anarchy. You will probably want to make a government action that automatically causes the country to collapse immediately once it reaches 100% infection (like the Shadow Slaves).

This also means you can increase a country's combat potential by increasing its Public Order through Government Actions or custom Events.

Since this engine uses Simian Flu variables, it does not work with the Simian Flu. (I picked the Simian Flu because it has a large number of possible variables, and it already has a combat system so there's probably less interest in using this engine for a Simian Flu variant anyway.)

It will work with the Necroa Virus, but it will do weird things. Only living infected will use the combat script, and some of the people they kill will turn into zombies.

It will also work with the Shadow Plague, but the combat calculations will be added on to the attacks from the Shadow Slaves. If you incorporate the Monster Plague Engine into a Shadow Plague, you will probably want to replace Shadow Slaves with custom combat abilities.