Narrative:

I was on first break for approximately 30 minutes when a flight attendant informed me of an odor/fume event and that the captain wanted me to return to the flight deck. By the time I was back up front; the captain and first officer had already made communications with ATC and dispatch to return to ZZZ. During this time; communication with the cabin crew continued. The captain asked me to go back and check on the odor and crew. I was told the odor was concentrated on aircraft right; by the mid lavatory. As I walked towards the rear of the aircraft; it was as though I walked into a bubble of gas. Approximately 10 feet more and I was out of it. The smell was very strong and concentrated; fitting the descriptions in our QRH; [though] I would not categorize as transient (during cruise; descent and continuing until past sundown at landing). I would also categorize it as a strong category a (sweet; sour; musty; dirty sock smell; irritating to the eyes). All of the cabin crew complained of similar symptoms (headache; nauseous; dizziness). Upon returning to the flight deck; I briefed the captain and first officer. I used periodic oxygen for the remainder of the flight as I too had some mild symptoms. The #1 flight attendant relocated several of the passengers. With symptoms continuing and the odor not diminishing; we decided that it was safer to land at ZZZ1. A medical emergency [was advised with ATC] and an overweight landing was executed to [runway] xxl. Some observations were upon arrival at the gate; several emergency vehicles were inside the aircraft area. The most disturbing thing was that there was no urgency to get the passengers and crew off the aircraft.

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Original NASA ASRS Text

Title: A330 First Officer and four flight attendants reported a fume event during flight as well as experiencing health symptoms resulting in an uneventful divert. Entire crew received medical treatment after landing.

Narrative: I was on first break for approximately 30 minutes when a Flight Attendant informed me of an odor/fume event and that the Captain wanted me to return to the flight deck. By the time I was back up front; the Captain and First Officer had already made communications with ATC and Dispatch to return to ZZZ. During this time; communication with the cabin crew continued. The Captain asked me to go back and check on the odor and crew. I was told the odor was concentrated on aircraft right; by the mid lavatory. As I walked towards the rear of the aircraft; it was as though I walked into a bubble of gas. Approximately 10 feet more and I was out of it. The smell was very strong and concentrated; fitting the descriptions in our QRH; [though] I would not categorize as transient (during cruise; descent and continuing until past sundown at landing). I would also categorize it as a STRONG Category A (sweet; sour; musty; dirty sock smell; irritating to the eyes). All of the cabin crew complained of similar symptoms (headache; nauseous; dizziness). Upon returning to the flight deck; I briefed the Captain and First Officer. I used periodic oxygen for the remainder of the flight as I too had some mild symptoms. The #1 Flight Attendant relocated several of the passengers. With symptoms continuing and the odor not diminishing; we decided that it was safer to land at ZZZ1. A medical emergency [was advised with ATC] and an overweight landing was executed to [Runway] XXL. Some observations were upon arrival at the gate; several emergency vehicles were inside the aircraft area. The most disturbing thing was that there was no urgency to get the passengers and crew off the aircraft.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.