Here in Cusco we´re high - as in 3,399 meters (11.152 feet) above sea level; other destinations in the Peruvian Andes not quite so much: for example, Machu Picchu 2,430m (7,972 ft.), Pisac 2,980m (9,776 ft.), and Urubamba 2,870m (9,416 ft.). And one issue that some (by no means all) visitors from lower-lying places can face is what we call soroche or mal de altura - altitude or mountain sickness provoked by the lower air pressure and oxygen levels up here. It can kick in at heights starting at 2,400-2,500m when for example they arrive suddenly by plane instead of getting more gradually acclimated by coming up here more slowly by road or train.
We locals are used to living so high up, but those who aren´t may experience symptoms including fatigue, shortness of breath, insomnia, persistent headache, elevated heart rate, dry cough, nosebleeds, nausea, and lack of appetite - and even being physically fit won't make you immune. The good news is that these symptoms usually disappear after one to three days (although it´s also true that in very rare, extreme cases dangerous cerebral or pulmonary edemas can occur). Fortunately, there are ways to lessen the likelihood of suffering soroche, or if it does hit you, to lessen its unpleasant effects.
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OMG, this brought back memories of my last visit to Cuzco, when I stupidly did exactly what this article says not to do: gave into temptation and had a big, scrumptious Andean meal. The walk back to the hotel was just a couple of blocks, but it was uphill, and soroche had hit so immediately and severely that I was literally crawling up the street with a pounding headache. Fortunately, it was gone by the next day and I was able to enjoy my next three days in the Andes. But oh, man...