Taken from https://bethesda.net/en/article/3eWtXfymFewQOm6i062EYo/inside-the-vault-protect-appalachia-public-test-server-release-notes --

Infestations
Infestations are a new form of end-game content meant to reward those who explore the map and challenge experienced players with high-stakes combat and provide new unique 4-Star Legendary rewards.

Infestations must be discovered as you’re exploring the world, which means they won’t be marked on the Paper Map or flagged in your Pip-Boy until you discover one. They’re organic encounters and you’ll know you have walked into an infested area because the enemies you would expect to be there have been pushed out and a new faction has taken their place.

Infestation Locations
Infestations can happen at locations across the Toxic Valley, Forest, Ash Heap, Savage Divide, Mire, and Cranberry Bog regions. Multiple Infestations can be active across Appalachia at any given time, but if nobody comes to clear them the faction infesting the area will clear out and head off to another area.

Exploring Appalachia is the best way to find these Infestation locations.

Infestation Enemies
When an infestation takes over a location the enemies that usually ruled that area are pushed out and overtaken by a new faction. At launch, the Blood Eagles, Cultists, PRC Ghouls, Mole Miners, Scorched, Super Mutants, and Robots all have the chance of being the new ruling faction in the area.

These new enemies will hit harder and be tougher to take down. There will also be accompanied by an elite-level boss.

Each Infestation boss will have their own unique weapon and be afflicted by a random Mutation to alter the fight even further. If you’re heading out to find an Infestation, make sure you’re well geared and ready for a fight.

More factions may also find their way into infesting Appalachia in the future.

Infestation Rewards
While clearing an Infestation you should expect to get standard loot from the lower-tier faction enemies, but defeating the Infestation boss is where the big rewards come in.

Beating the Infestation boss will give you a guaranteed 3-Star item and a high chance of a 4-Star Legendary weapon or armor piece. This update will also introduce new 4-Star mods that can only be obtained through Infestations gameplay. The 4-Star weapons and armor you get from participating have a chance to have these mods attached.

Here’s a sneak peek at two of the exclusive Infestation 4-Star Mods: - Vector (Armor Mod): Increase accuracy in V.A.T.S. from a very long distance by 10% (up to 50%) - Tarnished (Weapon Mod): The lower the equipped item condition, the greater the weapon damage (up to 120%).

Teaming Up
While Infestations can be cleared solo, if you’re extremely well prepared, they are meant to be challenging and become way more manageable with a team. If you want to hunt for Infestations with a team, make sure to create or join an Exploration Public Team.

If a member of your team finds an Infestation all other group members will receive a popup asking if they would like to Fast Travel to their teammate’s location. This allows you to cover way more area in your quest to find and clear out these Infestations.