Altitude question for fellow flat landers that have headed west

[BigPine]

I have friends that have went off and on since the early 80's , no problems with any of that group until one got pretty sick and had to go down to town at about 7500 feet. We live at about 300 ft elevation in Indiana and camp at 11,000 ft. I have been twice with no problems, other than I have a really hard time sleeping there. Drink lots of water and take an advil a day. I prefer to get there a day ahead and get a bunkhouse for a night before packing in.