The Creeper is a virus in the story of Advance Wars: Days of Ruin, which is also called The Green Thumb, Creeping Derangea, and Endoflorescens terribilis. It is a large plot device and recurrent theme throughout the game. It causes plants to begin growing inside one's body, eventually resulting in flowers blossoming out of the body and death.
Creation and the Meteors
The Creeper was invented as a bioweapon by Caulder; it was devastating and had nearly no symptoms until the blossoming at one's death, but he rejected it as a failure because its inherent photophobia made it unable to survive in sunlight. However, once the meteors struck, the entire planet was covered in dust, preventing the sun's rays from striking the surface and creating an ideal ecosystem for it to live in.
Days of Ruin
The virus is first heard of when it is described by Dr. Morris once the 12th Battalion encounters a field full of the flowers; he says that the virus had not yet infected anyone over the age of twenty, for reasons unknown. Soon afterward, Isabella contracts the disease and is considered incurable until she makes a miraculous recovery at the shelter. It then fades out until a mysterious mutiny occurs within the ranks of the New Rubinelle Army just as Greyfield was about to crush his enemies. Several captured soldiers reveal that the virus had began spreading like wildfire throughout the ranks, making it clear that the virus had adapted to infect anybody regardless of age. Upon being scanned, over half of the battalion is infected. Morris is able to develop a vaccine for the virus, and is making progress towards a cure, when he discovers that all nearby medical facilities had been looted by Greyfield so he wouldn't become infected. The 12th Battalion manages to defeat the NRA and take the vaccine, distributing it to all the uninfected, and Morris manages to cure all the others.
It is clear that the virus had very few noticeable symptoms. Although Will and Brenner were able to tell when Isabella was infected, the virus soon lost many of its traits. After the discovery of the virus' infection of grown soldiers, its only visible symptom before death is a black diamond mark that appears on the skin shortly before the blossoming.