Infected

Infected are enemies in Estranged: Act I and Estranged: The Departure. They are humans, presumed to be previous inhabitants of the Island.

It is not explicitly revealed how humans end up infected, however some scenes heavily imply the cause to be on the island.

The creatures are violent, but not intelligent, and are generally easy for the Fisherman to dispatch.

Origins

The Wentworth Herald Issue #317 provides some hints about the origins of the infection (the newspaper edition is available in both games):

The colonies of algae situated on the island remain strong, as recent surveys show a vast increase in the amount of so called "red algae", the island's very own unique species of algae continues to take over lakes, and other natural bodies of water.

Local resident Rosie Hardman considers the algae a menace, and would be happier if it were gone, saying "It [the algae] is taking over, we need to get rid of it as soon as possible. I don't want all of the water on the island to be stagnant. What is it good for anyway?"

This article outlines an explosion in the Red Algae population. This is innoculous on its own, but the Liam Jones Diaries from Estranged: The Departure provide more explicit clues:

This is the uh, Liam Jones, audio diary, #1, we just started studying the algae, and we realised that it actually glows if you leave it alone for a little while.

This is Liam Jones, the live sample security engineer on-site - we've just admitted. Dr Frederick Prezella, into the observation room, uh, he became violent and attacked one of my members of staff so we're going to keep him under observation and probably move him off the island as soon as we can. We're not sure what caused this.

Possible algae pool found following the Archaeologist Diaries

These two events again don't provide a conclusive link, but it's a hint that the study of "the algae" lead to an unexplained violent episode. A further piece of the puzzle is provided by the Archaeologist Diaries in The Departure:

The pirates and sailors were involved in some kind of internal conflict. We were assuming it was over the precious artefacts that we recovered from this cave. We think that they were likely inebriated at the time, and there are bite marks on some of the bones that appear to be human.

We had to extract some of the team, as they fell ill. We have to assume they disturbed something near the bodies. We're going to be using our HAZMAT suits from now on. We know it's not safe.

There are no further audio diaries, and while it doesn't mention the algae explicitly, if the substance which was disturbed was red algae, it would be consistent with the account from Liam Jones. Slightly further on from the cave in which the audio diaries are found, a red substance which could be algae can be seen, providing another hint of a link between algae and the infection.

In Arque Wentworth, further analysis is visible in the form of projected slides, which confirms that:

The expedition was organised by Glasgow University
The group was investigating pirate-era artefacts and were infected by "algae contaminants"
The cover-story is that a ship sunk (presumably the ship the group was travelling on)
Sokol-Glockner were involved in the "clean-up process" and two personnel were lost
The area has been quarantined (though the Fisherman is able to access and traverse the area)

It also shows that Arque has embedded itself in the local government, to prevent future approval of the kind of permit used by the expedition.

Spread

The final scene of Estranged: The Departure, depicting an infected human approaching a corpse in Arque Arctic.

The spread of the infection is not clear; it is implied during the interrogation of the Fisherman by Sokol-Glockner personnel that Arque believe they are protecting the world:

Why are you here? You think you can change what's happening? We're protecting the world, don't you understand that?
This is all part of Arque's very necessary precautionary plan, one that you will NOT be interfering with.
Thousands, millions, billions of people. You have no idea what we're protecting here. The "greater good" doesn't quite cover it.

Thus the infection has not left the Island; or at least has not reached much of the world's population.

The final scene of Estranged: The Departure, set in the Arque Arctic facility depicts an infected human approaching a corpose. This shows the infection has spread to at least this facility, casting doubt on the claims by the Sokol-Glocker guard.

List of appearances

Estranged: Act I (First appearance)
Estranged: The Departure