User blog comment:Evil117/Halo vs Half-life: Gordon Freeman vs The Flood/@comment-211.26.90.202-20130602063826

The flood AKA the precursors could of easily infected Gordon with the act of flood spores, in order for Gordon to survive he needs a helmet or respirators to prevent such infection.

furthermore unlike headcrabs, the flood are not stupid and they would find a way to infact him, even if he had superior weaponary (Go ask the Forerunners :p)