Sickos

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.

In When the Cold Breeze Blows Away, they all serve for the South Vietnamese Umbrella Force since the prototype Kẻ Thù Virus is created by Nguyễn Văn Thiệu, but most of the sickos are killed while the remaining are captured, dying or wounded by immunized grownups, their children and many heroes (like Leon Kennedy, Su Ji-Hoon, the Preschool Girls, etc.