At altitudes above 3,000 meters, mild symptoms of altitude sickness are common.
I've spent a week at that altitude - with excursions on foot up to around 3800m - with no ill-effects at all. On the other hand, my room mate was barely functional at 3000m.
Another time in the mountains (this time in the Alps) I was hiking while carrying a (lesser) load at around 3880m. There were six of us in that group and two did have serious problems up there, the stupid thing was that they were aware beforehand that they were susceptible - apparently around one third of the population is.
Some people need time to acclimatise to altitude, but I don't understand why people who should know better think that even marching up the foothills is a good idea. In my case I have never been higher than just under 4000m and have never had altitude sickness so I don't know what my limits are. Maybe I should head up Kilimanjaro (just under 5900m) but I think I'll pass, and maybe Everest tourists should be required to walk up Kilimanjaro - or an equivalent - before tackling the big one.