The Infected are people who have recently been bitten by a full Vampire. They then spend two days as a mindless minions until they complete the change into a full Vampire. Just like normal Vampires, they suffer a fatal allergic reaction when they swallow salt water.
In the Books
In the second book, Valkyrie is nervous when her Reflection is not where it was supposed to be, despite leaving before her. Worried, she goes looking for it and runs into Dusk and two Infected, a man and woman, who he called Minion One and Two.
Also, Dusk uses the army to try and kill Valkyrie. They track her to her family reunion and chase her through Haggard. Valkyrie jumps off the Pier, and is followed by the Infected, who are unaware of their weakness to salt water. Their throats swell and they soon drown in the ocean. At the end of the book, Valkyrie is by the Pier, on look out for any Infected the water didn't kill. Skulduggery arrives and tells Valkyrie to get some sleep. She says if nothing jumps out at her by the next day, she would. Just as the partners go to leave, they feel the air currents change. Skulduggery and Valkyrie turned and see an Infected, battered and broken but still alive and hungry. Skulduggery took out his revolver and Valkyrie summoned fireballs, preparing to fight it.
In the middle of the book, Skulduggery, Valkyrie and Caelan visit Moloch, a Dublin vampire living in a Ballymun flat. He receives people for food and to infect in exchange for protecting the inhabitants against the drug dealers and drunks. Skulduggery and Valkyrie believe Moloch would know where Dusk is but he turned on everyone, attacking Skulduggery and Caelan and throwing Valkyrie into where he holds the Infected because, even through killing all the Infected did him a favour, she had killed his brethren. Valkyrie was attacked by a pack of Infected until she found one Infected by Dusk. He was unconscious as she had burned him with a fireball. She received the information and asked to come out, fearing for her safety as the Infected were growling at her. A hand lifts her out. Valkyrie thinks it's Skulduggery's but it's actually Dusk and he throws her off the top floor of the flats. Skulduggery saves her before she dies.