The flames broke out on the third floor of Masten Towers, which houses low income seniors and people with mobility impairments.
Crews rushed to the scene and contained the fire to the room where it started.
One man was taken to the hospital with severe burns.
Another person had smoke inhalation, but was treated and released at the scene.
Fire officials tell Action News three sprinkler heads helped save the 11 story building and possibly hundreds of lives.
"Not only did they keep the fire in check but also reduce the smoke, and we know...injuries, including deaths in fires, and with the amount of people in this building, we would have been facing a disaster," Fresno City Fire Battalion Chief Tony Escobedo said.
Some residents were evacuated during the fire, and others were told to shelter in place.
The cause is still under investigation.