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:
[ Your endurance increased by 2! ] [ Your nanite healing resistance increased by 60! ] - This makes nanite healers less useful. [ Your generic disease resistance increased by 100! ] - You will be immune to the vast majority of diseases. [ Your radiation sickness resistance increased by 50! ] - This gives you a 50% chance to shrug off rads that you'd receive from walking around in radioactive areas. [ Your alien technology psychosis resistance increased by 60! ] - You have a 60% chance to shrug off ATP doses from wearing deathgear.
If no longer being susceptible to disease/sickness isn't good enough, you're also immune to vampirism and zombie rot. The +2 endurance cannot be overlooked either, especially if you're a spear/focus user. Note that you are not protected from toxin exposure, such as drinking contaminated water or the Toxic Dump.
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.
Hyperimmune raises endurance, which can make it harder to get decent dosages from drugs such as speed, crank, or cocaine. This may cause players to still receive the addiction effect without actually receiving any dosages.