In our group of nine, seven of us battled some type of illness on the way up (and down!). The two that didn't get sick were Sherpas who have lived on the mountain their entire lives and trek to base camp for a living. If you do everything right — drink tons of water, take it slow, have rest days, etc. — altitude sickness can still hit anyone, including experienced guides who have been doing this for years. It doesn't matter how fit you are; there's no rhyme or reason for who can get altitude sickness. This can present itself in the form of headaches, nausea and vomiting, loss of appetite, fatigue, and extreme shortness of breath. In our group, my brother-in-law caught a stomach bug, I battled debilitating headaches, someone else lost their appetite (and their energy), and others had to hike through bad waves of nausea.
That fact can be daunting and scary, but again, you have to listen to your body. This might mean staying behind for an extra rest day or, worst case scenario, turning around and going back down the mountain if your symptoms don't improve. The worst thing you can do is continue to go up if you're really not feeling well. That can result in altitude sickness progressing into something more serious, like HAPE (High Altitude Pulmonary Edema), or a buildup of fluid in the lungs, or HACE (High Altitude Cerebral Edema), when there's fluid in the brain, which are both life-threatening.
Image Source: Caitlin Gallagher