Passage: Everest base camps refers to two base camps on opposite sides of Mount Everest (It could also include any Everest base camp on a given route, but this is less common since the two main routes became standardized). South Base Camp is in Nepal at an altitude of 5,364 metres (17,598 ft) (28°0′26″N 86°51′34″E), while North Base Camp is in Tibet at 5,150 metres (16,900 ft)  (28°8′29″N 86°51′5″E).  These camps are rudimentary campsites on Mount Everest that are used by mountain climbers during their ascent and descent. South Base Camp is used when climbing via the southeast ridge, while North Base Camp is used when climbing via the northeast ridge.Supplies are shipped to the South Base Camp by porters(Kulli), and with the help of animals, usually yaks. The North Base Camp has vehicle access (at least in the summer months).  Climbers typically rest at base camp for several days for acclimatization to reduce the risks and severity of altitude sickness.

Question: Climbers are at risk for what type of illness?
altitude sickness