Soroche, or mountain sickness, is a reality for most people arriving in Cusco by plane from sea level and needs to be treated with respect. It’s vital to take it easy, not eating or drinking much on arrival, even sleeping a whole day just to assist acclimatization (coca tea is a good local remedy). After three days at this height most people have adjusted sufficiently to tackle moderate hikes at similar or lesser altitudes. Anyone considering hiking the major mountains around Cusco will need time to adjust again to their higher base camps.
If you do encounter altitude-related health problems, many hotels and restaurants have oxygen cylinders to help; alternatively, for serious cases, try the Clinica Peruano Suiza (English spoken) at Calle Meson de la Estrella 168 (open 24hr; t237009, wclinicaperuanosuiza.com), which also has a dedicated medical network whose details can be accessed at wo2medicalnetwork.com, and the Clinica Cima at Av Pardo 978 (t255550).