At high altitude. Only people climbing to a high altitude get high altitude illness, especially if they do it without proper acclimatization.
Common. Almost 1/3 of people have symptoms above 3000 meters (9800 feet) some seem more sensitive at lower altitude. As said, going up faster, (also sleeping high at night, perhaps not being fit, not using preventative medicine) all increase risk. Altitude illness is first reversible by reversing direction and going back downhill. If ignored--deaths do occur. Take it seriously and learn about symptoms.