I can only concur with what most others have said (I have experience up to 14,000ft).
If you don't want to slowly gain elevation you can go straight in at 8,000 but take measures to avoid altitude sickness. First two days, do nothing. Drink lots of hot/warm water. After that, take it slow and just ease into it. You'll probably notice yourself get out of breath easily when exerting. Make sure to be drinking at least 2 litres of water per day if not more. Hydrate as much as you can before sleeping because even if it means getting up at night to pee, that's better than waking up with a banging headache and spending the first two hours of the morning getting with it. That only happened to me camped at 14,000 though. 11,500 was no problem.
Should you become really sick, just go down. I'd consider that quite unlikely though unless you went a lot higher and started running around immediately.
P.S. Water takes longer to boil at higher elevations.
Yeah, I can relate. Where I am at in Yellowstone is 7,800 feet---I'm at Canyon Village (Wyoming) and I'm working at Yellowstone Adventures. When I first got here, it was around 5:30 am at West Yellowstone, MT. I had maybe a couple of hours of sleep because I had to report to HR at 8 am. I drove all the way from Sioux Falls on Easter Sunday to West Yellowstone since I wanted to spend the holiday with my daughter before leaving out. However, not getting enough sleep was not a good idea. I was so tired once I was at my seasonal location. I also got sick because I was not used to being up that much in elevation. Once I got to where I was, I just went to sleep but had a horrible headache. The people already here told me to drink lots of water and especially so before going to bed. Once I listened to their advice, I felt better. I guess it will take me some time since I have never been anywhere other than Florida and South Dakota as an adult. It's better now for me, though. I really enjoy it here.
I have taken some cool pictures with my smartphone. SIGH! I have to save up and buy another computer as mine took a crap on me. I'm using the employees' computer in the common area in the dorms right now. The Internet is quite slow here and is not very reliable. It is what it is.
VanGrrl57