Senior Citi-zombies are a form of undead created by Grandfather, who can transform people into them through touch. When turned into one, a person becomes extremely old and decrepit to the point of being corpse-like in appearance, with other zombie-like qualities such as yellow pupil-less eyes. Like Grandfather himself, Senior Citizombies can turn other people into their kind just by touching them.Senior Citi-Zombies appear to be very powerful and apparently unkillable, as shown by the zombified form of the normally harmless Toiletnator. Victims of zombification do retain elements of their personalities, but lose their free will and become Grandfather's evil slaves regardless of who they are.
The first people turned into Senior Citizombies were the villains at the Convention Center, starting with Toiletnator. This already-sizeable army then proceeded to invade all the KND Treehouses, zombifying everything in their way in the process, until the entire world was under Grandfather's control, eventually even seizing the KND Moonbase,. The people of Earth were only returned to their normal selves when Grandfather was defeated and decommissioned by Numbuh 0 and Sector V.
Age-o-tonic Birthday Suits were modified by Numbuh 2 to prevent zombification after the epidemic began, but were only partially complete by the time Numbuh 2 himself was turned into one. When Numbuh 1 put his team into the suits near the end of the film, their bodies are only partially covered and the parts that aren't are still zombified and still evil.
An odd thing to note is that very close friends of Grandfathers relatives, notably Sector V, and Numbuh 362, take some time to be transformed, as rather than an near-instantaneous transformation seen in the villians and normal KND Operatives, they require time to be transformed, Numbuh 2, 4 and 362 all being examples of a resistance to the transformation. Numbuh 2 is shown to be zombified slowly, Numbuh 4 screams for a significantly longer time than normal operatives, and Numbuh 362 required multiple zombies (at least five) to hold her down to complete the process. Although Numbuh 5's transformation was quite instant most and Numbuh 3's transformation was not shown.
It is shown that the Zombification process is slowed by having the Book of KND, or completely halted if the zombie's hold is broken, as shown with Numbuh 0.
Despite being on the Earth at the time of her Zombification, Numbuh 3 can be seen climbing the window of the KND Moon base.
Even though zombification makes a lot of noise, flashy lights, and typically screams of terror, some transformations were completely silent, Examples would be Numbuh 3 and Numbuh 86. when Numbuh 3 get changed by The Hamsters and/or Numbuh 5, and when Numbuh 86 is changed in a closet with Numbuh 2.
- Numbuh 1 , Numbuh 14 , Heinrich von Marzipan , Lizzie , Mushi , Teen Ninjas , Maurice , Madam Margaret, Boys Next Door, Jerry Rassic, Vin Moosk, Cheese Shoguns, The Kid, Eggbert Eggleston, Joe Balooka, Willard Wallace, Wilson Woodrow, Six-Gum Gang, Senator Safely, Muffy Jenkins, Principal Smelling, Mrs Thompson, Mr. Frybingle, Gallagher Elementary Students, Numbuh 1's mother or any of Sector V's parents, Cuppa Joe and Chad were the only characters who didn't get turned into Senior Citi-zombies. - You don't know that!
- Looking closely, when an operative gets zombified, they simply gain a greyer variant of their outfit, in additon to the usual orange pupilless eyes and wrinkled skin. This is easily seen on the operatives with the yellow and black outfits with cross-box helmets. One (A girl) is seen being zombified in the background by an Ice Cream Man while Numbuh 2 attempts to contact Numbuh's 3 and 4, and two more are on the top level of the base, one being carried to his zombification, and another attempting (unsuccessfully) to get out of another zombie's grip. All end up with the same effect, grey and green armor with wrinkled skin, and for the girl, grey hair.
- None of the moonbase exclusive operatives are seen zombified, implying they were able to fight the zombies off with more success than the other operatives.
- Several operatives are seen being attacked by the zombies, but it is unknown if they were zombified. Notable examples consist of Numbuh 35, Numbuh 23 and an female operative on the stairs, along with a JKND operative.
- Numbuh 0 is the only operative who has been hit with the zombification process, and not been zombified. This is due to the fact the Book of KND was on him at the time and he is Grandfather's son.
- Numbuh 1 is the only operative of his team to not be zombified. Furthermore, he is the only operative outside of Numbuh 0 and Sector Z to engage the senior citizombies in hand to hand combat and not be zombified.
- There is a zombie who is standing around while an Moonbase operative walks past him, hinting that zombies sometimes have bouts of amnesia, and forget that they are to attack and zombify the KND.
- Numbuh 3's eyes were not yellow and had pupils when in the closet with Numbuh 4!
- Somehow the Zombies managed to get to and from the moon quite constantly, yet no showing of it or them flying there or anything.
- Numbuh 4's shrieks of horror during his transformation just managed to slip past Numbuh 1 as he approches the treehouse.
- It appears treehouses only turn into tapioca factories when all KND member accompanying it are transformed Sector V's didn't turn or the moonbase.
- KND Arctic base was the only KND site and treehouse that didn't provide a battle/tranformation scene
- Sector V's Treehouse appeared to be under lockdown as shown in the episode Operation L.o.c.k.d.o.w.n but Tommy, Numbuh's 3, 4, and 5,and Tolietnator all got in and out.
- What became of babies and toddlers if zombification only worked on Kids and former kids?
- A lot of big villains were not showed or acknowledged as zombies such as, Count Spankulot, Mega Mom and Destructo Dad, Chester and Robin Food.
- The where abouts of where Chad had gonna or if he ever became a zombie were never revealed after he left in fear of ,as he put it, "getting all wrinklified.
- There is no reason as to why the Toilenator was zombified slowly while all other zombifications seem to happen instantly.