What is altitude sickness? Planes fly at 38,000 feet. Can you get altitude sickness?

Lets use a popular city for an example. We have chosen Denver, Colorado.

Its elevation is about 5,000 feet above sea level on average.

This next paragraph is taken from a reputable web site (Denver University website), and we have included a link to the site if you wish to double check any information. Please read the paragraph now:

If you are planning a trip to Colorado, be cautious of altitude sickness (dizziness, nausea, light headedness, headache, fainting, etc.) which can strike the most athletic people. The air is thinner at our altitudes, and the higher you go the more suceptible to symptoms you may be. A general rule of thumb is to give yourself 24 hours to acclimate to 5,000 feet above sea level. After that, give yourself 24 hours to acclimate to each additional 1,000-2,000 feet and your body will thank you. Throughout the acclimation process drink lots and lots of water because you will be dehydrated without even knowing it.

[Source: Denver University. http://www.du.edu/och/beyond.html]

So you aren't going to Denver, your safe from altitude sickness? - Wrong.

So hang on, I am in a plane, 38,000 feet above sea level - I know it is at high altitude, but they pump oxygen and stuff into the plane so you aren't 'really' at high altitude? - Wrong.

The plane is pressurized, correct. Oh good so I won't get altitude sickness - what a relief - I was worried about that.

Well you can start worrying again, because planes pressurize their cabins to an average altitude of 8,000 feet. So hang on, that is a further 3,000 feet above Denver, Colorado. That is quite a lot.

So now, we ask you to read the paragraph again. They explain it is best to stay 5,000 above sea level for 24 hours, then 24 hours to acclimatise to each additional 1,000 feet. Just go down to your local state universities phys. ed. department, or NASA, and they have pressurised training chambers. Stay in there for 3 days before going onto a plane. We ask you to do this for your own health and wellbeing.

It is ludicrous to think you can just go from a sea level city such as Los Angeles, to 8,000 feet and not receive altitude sickness. Once again, keywords in the above paragraph "... can strike the most athletic people."