Hyperimmune is located within the Phage Lab, deep within the Subway Tunnels and can be difficult to obtain. The area is swamped by stronger than average zombies, as well as tough robots. You'll need to wear a gas mask in the phage lab itself, or risk becoming infected with the (quickly) fatal disease. Be prepared to climb, scavenge, and use your brain to solve a few simple puzzles.
This mutation grants:
+2 to Endurance Immunity to disease 50% resistance to rad sickness Reduces the effectiveness of nanites by 60%
If no longer being susceptible to disease/sickness isn't good enough, you're also immune to vampirism and zombie rot. It's also needed to effectively wear the deathsuit, one of the best armors in the game. Note that you are not protected from mental illness, or brain damage. The +2 endurance cannot be overlooked either, especially if you're a spear/focus user.
Taking it does have is downsides though, you can not become a Vampire, or Zombie, if you were planning of going that route. Also nanite healers don't work so well, which is a pretty bad thing if you don't have a decent medic skill.