“It’s very easy for soldiers to get lost in high altitudes. The soldier felt dizzy and then lost direction, entering the Indian border,” Zhou said. “Both India and China are used to returning lost soldiers every year during scheduled meetings.”

A similar incident occurred last October, when PLA soldier Wang Yalong was returned to the Chinese side after two days in Indian hands. China said Wang had become lost while helping a local herder find lost yaks.

According to Zhou, it is common for PLA border soldiers to help herders find their animals, which cost up to 50,000 yuan (US$7,700) each. Their prized wool is used for expensive clothing. “But many Tibetan herders’ traditional grazing lands overlap with the Indian LAC,” he said.

Last September, China returned five Indian citizens who went missing from the border province of Arunachal Pradesh, a disputed region which was at the centre of the fighting during the 1962 war between the two Asian giants. Beijing claims the area as a large part of South Tibet.

Yogesh Gupta, a former Indian ambassador to Denmark and a specialist in China-India relations, said it was very common for both sides’ soldiers and citizens to lose their way along the 3,200km (2,000 mile) border lines, with many of them still in dispute.

“[There are] three problems: undemarcated borders, no signs of any sort in that area, and a lot of snow at this time,” he said. “Also, so many troops from both sides have come to live in those border areas for the first time.”

New troops and weapons have been deployed in the Himalayas by both sides since tensions escalated in June last year, when 20 Indian soldiers and an unknown number of PLA troops were killed in hand-to-hand combat in the Galwan Valley in Ladakh.

03:06

India pushes to build roads near Chinese border, in a bid to boost infrastructure in border areas

India pushes to build roads near Chinese border, in a bid to boost infrastructure in border areas

Elton Ng Chun-ting, a Hong Kong physiotherapist and the seventh Hongkonger to have summited Mount Everest, said most people would experience mountain sickness at over 2,500 metres.

“Mountain sickness symptoms include dizziness, headache, vomiting and other problems caused by low blood oxygen levels, which would also affect human cognition,” he said, adding that mobility also becomes “very limited” at such extreme conditions.

“Mountain sickness can cause acute cerebral and pulmonary oedema. Mild illness can be treated with medicine, water replenishment and good rest, but critical symptoms need emergency withdrawal to a lower altitude for treatment.”

China kits out army for winter combat as India border dispute drags on

Without efficient transport and communications support, altitude sickness could lead to deaths in the remote mountains between China and India, Ng said. All patients needed emergency treatment in one or two days, he added.

Wei Zhengjie, a PLA soldier stationed on the Karakoram – a snowy mountain range spanning the borders of China, India and Pakistan – died from an acute pulmonary oedema caused by altitude sickness on June 18, 2019. He had arrived at Tianwendian post just two days earlier, to take part in an outdoor long-distance running event.

His death was announced as part of a National Day documentary broadcast by China Central Television in October and was not an isolated case. According to the state broadcaster, dozens of PLA border soldiers have died over the past four decades from sickness, without giving details. But a source close to the PLA said all of them had been killed by altitude sickness.