It was meant to be replace the Sanity meter from The Dark Descent, and provides some of the same purposes as the Sanity system, providing visual and auditory distortions and hallucinations as the infection level increased. This would escalate to Mandus having to stop, retch and/or vomit as the player moved through the game environments. Enemies attacked the player through infectious damage rather than the physical damage that is present in the final game, and players were able to heal themselves through the use of the decontamination chambers in the game. However, this system would be more abstracted than the sanity system, removing the requirement for a gauge-based representation to track "infection level". Instead the player would be able to approximate the level of infection through the visual and auditory cues provided during gameplay. The linking of this mechanic to the attacks of the enemy agents and to the lack of cleanliness in the Victorian London setting was intended to blend the mechanical workings of the game to the setting and plot in a more integrated way.
As development continued however it was clear that the system simply was not integrating well into the rest of the game, and felt too much like a "mechanic" for the sake of being a "mechanic". The infection-based attacks from enemies for example, felt weak and unthreatening at best and downright confusing at worst. Similarly, environmental infection events, however they were framed, could not shake the feeling of players walking through luminous green toxic waste in any number of classic shooters. After many attempts at integrating the system more convincingly into the game, the decision was taken to remove it. This removal, while certainly not trivial, allowed much more focus to be paid to the core essence of the game - the story, and the environments through which it is told.
The removal of the sanity system was an important aspect of producing the game that TCR wanted to produce, and has been predominantly successful in doing so, as reflected in the writing of a number of critics that have praised its absence. While it may have been possible to continue with the infection system and build it into the game in a manner that felt more integrated, the decision to remove it allowed more attention to be given to other aspects of the game. The result is a game that is almost certainly more cohesive across its story, world and gameplay and thus is stronger as a whole. Without the level of trust and freedom provided by FG a change as critical as removing one of the core systems would have likely caused more serious concerns and delays in the development, especially as the system was such a key component of the originally pitched game.
Level 0: Default
Level 1: Light infection.
Level 2: Moderate infection.
Level 3: High infection.
Level 4: Lethal infection.
The Infection meter can be decreased by using the Penicillin (which decreases it by two levels) or Laudanum (which decreases it by one level).