The Health Department is tracking down 500 participants of a hike in Ma On Shan on August 28 after one of the hikers fell ill with the bacterial disease leptospirosis.
The man, 21, developed fever and chills on September 8 and was admitted to Prince of Wales Hospital on September 10.
A departmental community medicine consultant, Thomas Tsang, said the hike, organised by the Boys' and Girls' Club Association, had taken place during the patient's incubation period.
The patient had hiked with abrasions on his feet and walked through streams and ponds.
The department's Centre for Health Protection had contacted family members and the patient's four team members, but none had developed symptoms, he said.
Most human leptospirosis infections occur through contact with the urine of infected animals.