Air Carrier Flight Attendant reported a fume event during flight. After conferring with flight attendants the Captain elected to return to departure airport where a safe landing was completed and maintenance support was provided.

Narrative

I was sitting in my jumpseat on climb when I (Flight Attendant #4 (FA)) noticed a very strong rancid; sweat; locker room smell. I asked the #2 Flight Attendant if she smelt anything which she said she did. I immediately referred to my manual: odor and fumes section and began reviewing procedure and fume categories. Once determined to fall into a category (oil source); I called the #1 who also reported smelling the fume. The number one reported she would call the Captain. The Captain called back to myself and the #2 to gather more information; at which point I began to feel ill. The Captain reported he would go through his checklist; and I began to assess the cabin for potential smoke and sources. No smoke or sources were found. Flight Attendants continued to communicate I began to feel more ill; with weakness; headache; cough; throat pain; shaking; and slow cognitive processing. The Captain made a PA stating that we would be returning to ZZZ due to odor in the cabin. At that time the number 1 asked me to see if any passengers smelt the odor as someone in first class reported that smelt something; I asked the final rows in which XXD reported he smelt 'burning grease'. I wore a mask and breathed through my shirt but the fume/odor continued to be overwhelming. I got in my jump seat and preformed my 30 second review; following procedure; as we prepared for landing. Once landing we waited on the taxiway for several minutes before being brought to the gate. Upon the seatbelt sign going off I monitored and challenged to disarm the door and continued to smell the odor; the #1 and 2 completed all call and the passengers were informed of an aircraft change. I continued to feel ill. Once off the aircraft the pilots reported smelling burning; and maintenance also reported smelling an odor. I met with [a supervisor] who guided me in getting to a hospital to be checked due to presenting symptoms. I contacted [Company] and proceeded to the hospital and was assessed/treated.