Simmons becomes infected with an alien virus after exposing herself to a group of people that have come into contact with a Chitauri helmet.
Did You Know?
The Chitauri helmet had to be created from scratch because all the Chitauri soldiers seen in The Avengers
(2012) were purely computer-generated. See more
When Simmons shows Coulson the "brain cells", they clearly lack axons/dendrites, which are branchings from the cell walls that make neurons easily identifiable. Brain cells normally use these branches to produce tiny electrical impulses. The cells shown look like red blood cells. See more
This is the victim's brain.
It looks like a burnt baked potato.
References The Avengers
Agents of S.H.I.E.L.D. Theme Song
Written by Bear McCreary See more