Despite Bairns being classified as Combat Abnormal, their entity class actually extends them to the Infected tier, making them behave as Infected do.
Infected Mobs
General Spawning
Spawned when a living entity that has an infected counterpart dies with Mycelium Infection, in which they will transform upon death. This excludes Infected Scientists, which do not have an uninfected counterpart.
Spawn naturally in low light level areas (excluding Infected Husks and Infected Scientists). The group size and spawn weight depends on the Infected. Infected will not spawn in biomes with an internal temperature value at or below 0.2 or in the Deep Dark.
Spawned from Scents, though the specific Infected spawned depends on the Scent's list. This excludes Infected Adventurers and Infected Scientists.
Spawned from Challenge Watchers during various waves of a Challenge Raid. This excludes Infected Diseased Villagers, Infected Wandering Traders, and Infected Scientists.
Spawned from Vigils during various waves of a Vigil Raid. This excludes Infected Diseased Villagers, Infected Adventurers, and Infected Scientists.
Evolution
All Infected have the ability to Evolve. The following are their possible Evolutions:
Infected Human → Knight, Griefer, Braiomil, and Phayres
Infected Husk → Verdathorn, Jagdhund, and Scavenger
Infected Drowned → Bloater and Naiad
Infected Villager/Infected Diseased Villager/Infected Wandering Trader → Leaper, Slasher, and Spitter
Infected Witch → Volatile, Mephitic, and Gorgon
Infected Pillager → Howler, Stalker, and Brute
Infected Adventurer → Nuckelavee, Protector, and Gargoyle
Infected Scientist → Conductor, Chemist, and Inebriater
Tier-Unique Mechanics
Starvation
Not to be confused with the Starvation Effect, Starvation is a mechanic exclusive to Infected and Bairns. As soon as one of these Mobs exists in the world, a 5 minute 'starvation timer' begins. If an Infected or Bairn is freezing, their starvation timer will tick down twice as fast. If the Mob hasn't obtained at least 1 Evolution Point before the timer expires, they will receive the Starvation Effect, which slowly damages the afflicted Mob and is replenished until the Mob is killed or Effect is cleared. If at any point, an Infected or Bairn obtains a Evolution Point, their starvation timer will be paused permanently (as long as they keep any Evolution Points).
There are other ways for Infected and Bairns to reset their starvation timer. These methods, however, do not completely prevent the starvation timer from continuing to tick (save for Human Remains and Wall Remains). If a starving Infected or Bairn is within 1 block of Human Remains, Wall Remains, a Biomass Bulb, or a Drowned Lump, the Mob will consume it and their starvation timer will be reset, as well as the Starvation Effect being removed. While Human Remains and Wall Remains grant Evolution Points (stopping the Mob from starving altogether), Biomass Bulbs and Drowned Lumps give Kill Points, making them only a temporary solution. Alternatively, if a starving Mob receives Regeneration or Instant Health, the starvation timer will also be reset. This does not extend for as long as the Effect lasts; only the moment it is received is when the starvation timer is reset.
Upon death, for any block within 0.5 blocks of where the Mob died that also has an air block above it, there is a 90% chance per block of placing either a Small Mycelium Growth or a Big Mycelium Growth. There is an additional 30% chance that Human Remains are placed, with no cap, meaning it's possible for more than one Human Remains block to be placed. Furthermore, if the Mob starved to death near a block with a side exposed to air, it will place Wall Remains instead.
Shared Aspects & Capabilities
Infected are hostile to most non-Spore mobs, with the exceptions being Animals and Fish (unless Should attack Animals is enabled via the Config) and mobs included in the Mobs Not Targeted blacklist within the Config.
Infected inflict Mycelium Infection for 30 seconds when dealing melee damage (excluding Infected Witches).
Can perform short dashes in liquids to pursue a target.
Infected have a 5% chance to drop a Scent upon death unless they are freezing.
Infected can fall victim to Starvation.
Able to utilize Kill Points to grant themselves buffs.
Present a weakness to the cold.
Infected are influenced by Horde Mentality; they attempt to pair up with other Infected, Evolved, Hyper Evolved, Experiments, and Bairns.
Infected can be Linked.
Can be a maximum of 60 Infected in a world at any given time, managed by the Despawning System.
Trivia
Some Infected, such as the Infected Human, have a torso made up of many cuboids. This is because, originally, the mod creator wanted their stomach to expand with the amount of kills they had. This was scrapped, but the models kept their detailed torsos.
There's a noticeable pattern in the eyes of Infected, with undead Infected having a black sclera and villageroid Infected having a white sclera. Though, the Infected Wandering Trader breaks this pattern by having a black sclera.
Gallery
A large group of Infected.
Ditto.
A group of Infected up-close.