The Infected were normal men and women who contracted the Harran virus that spread through the entire city of Harran. They possess reduced higher brain function and attack and subsequently infect any uninfected targets around them on sight, mainly with their arms or with handheld weapons. The combat system in Dying Light allows the player to cut or break an Infected's arm or knock them down, which will prevent the infected from using that specific limb or limbs to attack the player.
The biology for common infected is similar to a human's. When you strike at the biters with a blunt weapon, you can see that their bones are structured normally proving that common infected are normal. The Harran Virus can and will alter the infected's biology when the sun goes down. The virus will increase their motor co-ordination and sensory capabilities making them deadly hunters. The Special infected are a different story, the Harran virus causes such severe mutations from the UV- vulnerable Volatile to the muscle-altered Demolisher that mutations must be either really slow to create something this mutated or the mutations are fast but use massive amounts of energy to generate new cells.
In Dying Light, the infected have a behavior that mainly depends on the "Day/Night cycle". During the day, the infected are sluggish and tedious, and also lack the ability to climb and run after the player which enables the player to move more freely around the environment. During the night, the infected tends to be more agile and stronger thus making it more dangerous for the player to roam around.