Dr Sophie Williams, 31, is on a life-support machine after contracting the disease Japanese encephalitis. Two in three sufferers either die or are left with permanent brain damage.
Dr Williams, a lecturer at Bangor University, North Wales, who has given talks around the world on conservation, has been in a coma for nearly three weeks.
She is in a stable but critical condition and her family describe her condition as “lifethreatening”.
She complained of headaches and nausea on July 6 while on a botanical research trip to China with students from the university.
Dr Williams fell unconscious in a remote area near the Vietnamese border, 400 miles from the Chinese city of Kunming, and was taken to hospital. Doctors identified the brain swelling disease but were worried they could not offer her specialist care and she was flown to a hospital in Bangkok.
Her parents Mike and Pauline and her partner Robert flew out to the Thai capital to be at her bedside.
Mr Williams, 65, of Malton, North Yorks, said: “We saw her come in to hospital. It was very distressing to see her that way.
“We know there’s a possibility that she might not make it which is a terrifying thought.
“This is every parent’s worst nightmare.
“We know this is a life-threatening condition but we are just praying Sophie can pull through. She has opened her eyes but she’s very tired.
“At the moment she is paralysed down her right side and doctors say part of her brain is damaged. She has no memory.”
Mr Williams, who runs a newsagent, has now returned home but his ex-wife and Dr Williams’s partner are still in Thailand.
Dr Williams lives in Bangor where she studied botany before receiving a masters degree from Imperial College London. She teaches conservation science and has been on other field assignments including in Gambia and Lesotho in Africa, and India.
Dr Williams was with three students in a small town when she was taken ill.
Up to 68,000 people are diagnosed with Japanese encephalitis every year, mainly in the Far East.
Sufferers usually live in rural areas and it is rare for travellers to contract it.