Altitude sickness crept up on me without warning. Four years ago a friend and I trekked through the Karakoram, the jagged western-most edge of the Himalaya that begins on Pakistan's Afghan border and sweeps into Kashmir.
Our first stop was the Shandur Pass (3,738m) which once a year is the venue for the world's highest polo match. Keen to get to the match as fast as possible we hired a jeep from the nearby town of Chitral (1,100m) and drove to the top of the pass to camp for the night.
The next day I could barely move without breaking into a sweat. My head felt filled with lead and my stomach was battered by waves of nausea. It was like being hungover and seasick at the same time. The only way I was going to get better was to drive back down the valley, meaning we missed the polo match entirely.
Slow ascents are vital. The next week we hit 4,200m without any problem because we trekked on foot. Earlier this year, the same friend and I climbed Ethiopia's Mount Bwahit (4,430m) and felt fine. Our guide, Fantabil, made sure we didn't rush. Only a week before he had to carry a seemingly fit 21-year-old British tourist who had collapsed. "He wouldn't listen to me," Fantabil said. "He thought he could jog up easily."