All Flood look like goo. Play Halo 3; they re-form when they take over a deceased body. Master Chief would be goo covered in/busting out of armor.
Because of his genetic augmentation and armor, it is impossible for him to become infected. Sgt Avery Johnson couldn't become infected because of augmentation as Spartan-I.
No, Johnson couldn't become infected because he suffered from Borreyn's Syndrome (spelling) which screwed up his nervous system just enough to keep the Flood from taking control of him.