"Sickos" is one of the names for grown-ups that have become infected with The Disease since The Dead. Though finding intelligence in them is rare when sickos do have it, they become very dangerous - for example, a sicko in The Fear was able to wield a knife. Most sickos, however, are not intelligent as the disease has left them with just the instinct of survival. Sickos will generally target children over other animals.
The main sicko of the series is St. George, who is intelligent enough to lead the sickos in organised attacks.
Having had both their minds and bodies ravaged by the disease, the sickos have become horrifying creatures covered in boils, blood, mould, mucus and pus often with injuries - such as "Pez" the sicko in The Dead who has had one of his cheeks ripped off. Despite having become crazed, child-munching animals the Sickos have retained some of their human characteristics. They are often strong, fast or smart but this is coupled with either slowness, stupidity or weakness so few are a danger to kids in small numbers.
How badly the disease affects an adult depends on their age and health. The elderly, decaying ones can barely move and shuffle along the streets at night, whereas teenagers, such as the ones who attacked Ed and Bam at the Fez, had hardly changed physically only gained the lust for children's blood which is needed to fight the disease. So far, all cases of gaining the disease seem to be slow enough for the infected to either acknowledge the disease like Fredrique or ignore it like Greg until it is too late.
Justin had summed up that sickos somehow needed something that is born along with children fourteen or lower years of age to fight the disease and live. The children had "something that prevents them from getting the disease" in their genes. As Justin said, "the longer they live the more the children they eat, and the more children they eat the longer they live." Somehow, the sicko's disease appears to accelerate when exposed to sunlight. They grow boils at a faster rate and their skin and muscles rupture in the day, so they move mainly at night, though some are still present in daylight. The sickos seem to mix the parasites memories with their own, like St. George and Wormwood did, thinking that they were "fallen stars" and "walked out from the jungle and became a man". Wormwood stated that "good blood will force out bad", and this is referring to Small Sam's blood which can be used to synthesise a cure for the disease.
St. George (aka Greg Thorne) - One of the last ones to get the disease. Leader of an army.
Spike - One of St. George's lieutenants. Shadowman shot him with a bolt when he was angry over the death of Tom and Kate. In The End, he wields two machetes and leads a group of sickos in Hyde Park, but is killed by Maxie.
Bluetooth - One of St. George's lieutenants. He wears a Bluetooth earpiece. He gets killed by a wounded, limping, very weak Shadowman, who chopped his head off.
Man U - One of St. George's lieutenants. He wears a Manchester United shirt. In The End, he wields two metal rods and leads a group of sickos to attack the museum, and is killed by Small Sam. However, several kids are killed on account of him and his group, including Whitney, Paddy, Zohra and Froggie. He personally kills Whitney by throwing a metal rod through her back.
The Collector - Fat sicko that collects kids. Then he eats them. He is killed by a group of Natural History Museum kids and hunters lead by DogNut, Ryan and Paul.
Pez - Minor sicko with a head that has lost its bottom jaw to look like a sweet dispenser. In the Battle of Lambeth Bridge, he is killed by Kyle when he attempts to bite Ed.
Wormwood - An ex-scientist and Fish-Face's father, he can still speak and was held captive by the group lead by Mad Matt.
Sentinels/Pointers - Sickos who stand still in a chain and communicate regionally for the army.
The Medusa/Tall Woman: A female sicko who leads an army from the countryside to London to join up with Saint George. Her army nearly kills Ed's group in The Hunted. In The End, they arrive in London on the night of the red moon, and kill Zulficker and Yo-Yo. In the final battle, she is killed by Shadowman.