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's 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."