The Italian cannibal film can be divided into two categories: zombies (Zombie, Night of the Zombies - the latter not to be confused with the Nazi zombie film of the same title), and non-zombies (Make Them Die Slowly, Grim Reaper).
Invasion of the Flesh Hunters features non-zombie cannibals - mortals compelled to eat human flesh by a virus that's contracted when one is bitten by an infected cannibal. Much like spreading lycanthropy, vampirism, or the murderous nymphomania in Cronenberg's They Came From Within (aka Shivers, Frissons, The Parasite Murders).
Invasion of the Flesh Hunters opens in Vietnam with the prolific John Saxon leading an assault on the enemy (NVA or VC, I'm not sure). A cheesy battle scene with extras running about aimlessly, flinging their guns while dying theatrically amidst fiery explosions. One enemy woman is set aflame in her cleanly pressed pajamas. All enemy pajamas look cleanly pressed and many things are set aflame, but mostly leaves. I don't think grenades can set tropical leaves aflame, but they seem to here, although there's also a flamethrower. Some of Saxon's troops carry M-16s, but Saxon holds what looks like an Israeli Uzi. The Vietnamese jungle looks like a Temperate Zone forest, and there's even a cave. The battle culminates when Saxon discovers two American POWs trapped in a pit - eating an enemy woman.
Saxon wakes up, nightmare over. It's been many years since the war ended. So why his persistent hunger for human flesh? Saxon's nightmare turns real when one of the POWs in his dream (and his former subordinate) phones with a request that they meet. Seems some vets contracted a cannibal virus in 'Nam, they're beginning to devour civilians, and soon the body count mounts.
The simple storyline follows Saxon's struggle to resist succumbing to his disease while aiding his infected comrades, all amidst the spreading rampage of flesh-hungry vets and civilians. Plot holes abound. Why does the cannibal nurse unstrap the cannibal vets rather than eat them in their state of helplessness? They're not zombies, after all; their flesh is still fresh albeit infected. And as in so many zombie films, one wonders why the cannibals only appear nibbled upon - why weren't they consumed more thoroughly when previously attacked? Nor is it ever clear where Saxon intends to lead his men, or why they're traversing the sewers. They certainly never get anywhere.