[agetreme]

Brain death....is also possible assuming that the whole decay thing is serious enough.

What do you expect? Our body is made out cell....
Hell, if you want to get technical:

Individual Humans are basically a country. A country of cells happened to be living and working together. If some important cells/people don't do their work or died without a successor, the whole country fall (you dies).

If too many of your healthy cells start dying suddenly.... you are screwed.

Blood are cells too... if red blood cells dies: loss of oxygen.
No Oxygen to brain, your brain dies too.....

Note: Based purely on speculation... for exact detail, you have to ask Zero for the gruesome stuff.

Edit:
I think I will make a joke out of various underground organization..... Unicorn is obviously referring to them but he can't be bother to remember their name so he called them:

Unicorn: Ruination of Virgin Industry - Panty Dusk.

Combine with Mana like telling ridiculous lies:
Apparently Ruination of Virgin Industry, is formed after world war 2 because there aren't many clean panties around. Their's goal(according to Mana) is to steal all panties around the world because it make them look less lame and more lesbian.
More ridiculous stuff: Panty Dusk are not actually lesbian but.... autophilia....who love chairs in an abnormal kind of way and a weird fetish fixture for panties.

Unicorn: Trashy Stuffy (Sarashiki Family)
Mana: Trashy Stuffy's goal is apparently trying to get rid of the world's trash.....but it is a difficult task because human produced too many trash daily.

Unicorn: It iS Academy (ISA)
Mana: Teach IS pilot how not to be stupid.

===
Fanon verse:
Unicorn: Sliver lining (Sliverback)
Mana: They are a bunch of old man trying to act like Gorilla.

Unicorn: Pixy Dust (Praxidike)
Mana: They believed in Fairies and try to collect Pixy dust from them.

Unicorn: Drag Empire (D.E.)
Mana*give funny look*: A bunch of immature kids pretending to be dragon while cross-dressing with their underwear inside out...