Wormwood, a.k.a The Green Man, is a character introduced in The Sacrifice. He appears to be covered in mould and may have an idea about how the epidemic started. He can murmur in broken speech, although it doesn't make much sense. You may have to decipher his ramblings. He seems to have a certain temporary power that he can use that freezes the sickos in place. He calls himself the fallen star, while the other half of him as he sees is Mark Wormwold from Promithios, studying tropical diseases. He has something to do with how the epidemic started and was one of the first of the people to get infected. He had three boys, a wife and a defective girl that was born just before the outbreak. He refers to the other sickos as brothers and sisters.
Wormwood first appears in the prologue, although only identified as 'The Green Man'. Later on, when The Kid is presumed to be The Goat, he attempts to eat The Kid but is persuaded otherwise by the latter. He then reveals himself to be Mark Wormold, a scientist working at Promithios. The Kid leads him out of the cellar where he is being kept captive and shows him to Ed. Whilst Ed is understandably uneasy at first, he, Kyle and the rest accept that Wormwood is useful. He appears to be able to communicate with other sickos, presumably telling them not to hurt the group. He goes back to Shadowman's safe house and is last seen with Kyle teasing him.
In The Fallen, he arrives as the Natural History Museum with Ed's group. Here, it is revealed that one of the mutated children from the warehouse, Fish-Face, is actually Wormwood's long lost daughter. They are reunited at the end of the book in the Natural History Museum.
In The End, Wormwood is seen working with Einstein as he attempts to find out more about the disease, however Wormwood's broken speech makes it harder for Einstein to get the information he needs. It's then revealed that Sam has an immunity to the disease and he Einstein take a blood sample and begin working on a cure. The first batch however proves unsuccessful on a mother they had locked in their Tesco lorry when she "bursts" however Einstein points out that the mother became capable of speech and then suggests they test their second batch of the cure on Wormwood himself in order for him to clear his mind and remember more about the disease. After St.George's death, Wormwood is last seen standing with his daughter contemplating his existence as the last adult in London. Maxie and Fish-Face have also pointed out throughout the book that the second batch of the cure seems to be working as the mould covering Wormwood's body seems to be disappearing, his speech has become easier to understand and he has shown no signs of hunger for children.