'We were at high altitudes for four days and I was ill the entire time'
Jessica Mathur, a GP from London, was surprised when she became ill with altitude sickness during a holiday in Peru.
"I was 19 and pretty fit when I went on a tour of Peru with two female friends. Like me, they were students who were looking for adventure.
"We arrived late in the day at the city of Cusco in the Andes mountains, 3,500m above sea level.
"While sightseeing in the town the next morning, I began to feel unwell. Even when walking along a flat street I felt quite breathless and unable to keep up with my friends. I vomited, had a bit of a headache and generally had to do everything extremely slowly.
"I found it difficult to believe that I had altitude sickness. I just didn't expect it would happen to me. I recognised what it was because it's in every guide book.
"I became quite grumpy because I knew I was holding the others back. I tried to just do things that took the minimum effort, but that didn't help. I had nausea the whole time and felt 40 years older.
"We travelled on by train to the lost Inca city of Machu Picchu, which is 2,430m above sea level. We were at these high altitudes for four or five days and I was ill the entire time.
"We weren't high up for the rest of the holiday, except during a hike in the Andes. My altitude sickness came back, which surprised me because we were in the foothills.
"I only had a mild case and didn't have any serious consequences, but I couldn't really enjoy my time at high altitude much.
"We didn't do any real climbing in the mountains. I didn't think it was wise to go up any higher. The altitude sickness didn't affect my friends and I found that annoying and a bit embarrassing because it just looked like I was very unfit.
"I told my friends I thought I had altitude sickness. The warnings say you must make sure other people know about it because there is a danger that your judgement can become clouded. Because of this, some people often resist the advice to go to a lower altitude when it becomes necessary.
"As neither of my friends were affected, I thought it would be hard for them to believe I was feeling really unwell, but they were very understanding.
"Nobody suggested I should go back down to a lower altitude. I wasn't so badly affected. I wanted to see the things we came to see and I felt lucky that the altitude sickness was mild.
"I haven't gone to a high altitude since then. I did have the opportunity to go up Mount Kilimanjaro in Tanzania, which is 5,895m above sea level, but I didn't want to go through altitude sickness again."
'Being careful to acclimatise properly did take extra time, but I was very glad I did'
After years of mountain climbing, David Hillebrandt learnt how to deal with his altitude sickness.
"In 1980, my wife Sally and I drove to Kenya from Britain as part of a world drive. I suppose you could have described me as a tough and rugged young doctor and an experienced climber. Sally didn't climb at all.
"Before my ascent of Mount Kenya (5,199m), a technically challenging rock climb, we decided to walk the little-used but magnificent high-altitude trek around the mountain to acclimatise.
"It was quite a humbling experience for me as we progressed along the beautiful trail at between 3,000m and 4,000m. Sally was happy and healthy and enjoying the wonderful flora as we crossed amazing ridges and valleys, but she watched me being slowly overtaken by altitude sickness. I was soon suffering from a terrible, severe, throbbing headache worse than any hangover, and vomiting up everything I ate.
"We planned a celebration for my 27th birthday, but all I could do was be sick. Sally must have been tempted to laugh at me, a great mountaineer reduced to a liability. I must admit, I did slightly resent her apparent immunity to the horrors of altitude sickness. We're just genetically different.
"I went down to a lower level for some relief from my aching head and enjoyed a good meal. That did the trick, and I was eventually able to climb the magnificent mountain in two days with no trouble. I couldn't have completed the rock climb if I'd been feeling ill. Being careful to acclimatise properly did take extra time, but I was very glad I'd done it.
"We got up to the summit in one day and dropped down about 100m to sleep tied to a ledge. Waking up to a fantastic dawn overlooking the African plains was something I'll never forget. It was certainly worth the effort, altitude sickness and all.
"Since then, I have become older and wiser, and I have learnt to go slower. I have climbed in the Himalayas and psychologically adapted to altitude sickness, but physically things are the same. It's still as bad as it was 30 years ago. The only difference is that now I know how to deal with it."