The movie you’re probably thinking of is The Crazies or Inmate Zero / Patients of a Saint , depending on whether the infected people are in a town or a prison setting. If you mean a prison-based horror where inmates and guards get infected and turn feral, Inmate Zero is the closest match.

Likely matches

  • Inmate Zero / Patients of a Saint : set in a death-row prison on an isolated island, with an infection spreading through inmates and guards.
  • Shapeshifter : a prison survival horror where guards and inmates face a monstrous threat.
  • The Crazies : not a prison movie, but it does feature an infection that turns people into aggressive monster-like threats.

What sounds most similar

If your memory is “prisoners get infected and become monsters,” Inmate Zero is the strongest fit from the results. If you remember more of a zombie outbreak inside a facility, Shapeshifter or The Crazies may be the one.

TL;DR

Most likely: Inmate Zero. Second guess: Shapeshifter.