Firefighters were called to 379 Charlotte St. just before 5:30 a.m. The fire was raging at the back of the building when they arrived.
"You could see flames coming through the doors and windows," said Mike Mizner, acting platoon captain with the Fredericton Fire Department.
"There was smoke coming from pretty much all parts of the house."
Platoon chief Ken Dickeson said firefighters armed with thermal imaging cameras found two residents right way and pulled them out.
They put out the fire and went back in about 20 minutes later for a secondary search. That's when they found a woman unconscious inside.
Paramedics gave the woman CPR. She was then rushed to hospital in Halifax to be treated for smoke inhalation.
Twelve people lived in the two-storey building, with 10 at the front and two at the back. One of three cats died in the fire.
The Canadian Red Cross says it's helping five tenants with emergency provisions such as clothing and food. All residents have found temporary places to stay.
The CBC's Jacques Poitras said residents were allowed back in Sunday morning to retrieve some possessions.
Cause of fire unknown
The investigation into the cause of the fire continues. Mizner said it appears the fire began at the back of the building.
The Red Cross says a smoke detector went off and alerted the tenants.
Resident Danielle Roy said she complained of a smell of gas in the last month. But Dickeson said he was not aware of any link with the fire.
Poitras said the owners of the building were watching the cleanup but declined to comment.