Acclimatization and altitude sickness

commonly called altitude sickness, this has the potential to affect all trekkers from 2500m and higher. Your body needs days to adjust to smaller quantities of oxygen in the air - at 5500m/18,044ft the air pressure is approximately half that of sea level, i.e. there is half the amount of oxygen (and nitrogen). This is approximately equivalent to the top of Kala Pattar, in the Everest region, and the top of the Thorung La on the Annapurna Circuit.
For treks below an altitude of about 3000m/10,000ft it is not normally a problem. AMS is caused by going up high too fast and can be fatal if all the warning signals are ignored. Note that it is not the actual altitude, but the speed at which you reach higher altitudes which causes the problems.

Altitude sickness is preventable. Go up slowly, giving your body enough time to adjust. These are the 'safe' rates for the majority of trekkers: spend 2-3 nights between 2000m/6562ft and 3000m/10,000ft before going higher. From 3000m sleep an average of 300m/1000ft higher each night with a rest day every 900-1000m/3000ft. ultimately it is up to you to recognise the symptoms, and only ascend if you are relatively symptom-free.
Normal symptoms at altitude

don’t expect to feel perfect at altitudes of more than 3000m. These are the normal altitude symptoms that you should expect BUT NOT worry about. Every trekker will experience some or all of these, no matter how slowly they ascend.
Periods of sleeplessness
the need for more sleep than normal, often 10 hours or more
Occasional loss of appetite
Vivid, wild dreams at around 2500-3800m in altitude
unexpected momentary shortness of breath, day and night
Periodic breathing that wakes you occasionally - consider taking Diamox
The need to rest/catch your breath frequently while trekking, especially above 4000m
Your nose turning into a full-time snot factory
Increased urination - many trekkers have to go once during the night (a good sign that your body is acclimatizing:
Mild Symptoms

You only need to get one of the symptoms to be getting altitude sickness, not all of them.

Headache - common among trekkers. Often a headache comes on during the evening and nearly always worsens during the night. Raising your head and shoulders while trying to sleep sometimes offers partial relief. If it is bad you may want to try taking a painkiller: aspirin (dispirin), paracetamol, Ibuprofen (Aduil) or acetaminophen (Tylenol). Never take sleeping tablets. You could also take Diamox: see below. Headaches arise from many causes, for example, dehydration, but if you develop a headache assume it is from the altitude.
Nausea (feeling sick) - can occur without other symptoms, but often nausea will develop with a bad headache. If you are better in the morning take a rest day, or if you still feel bad descend.
Dizziness (mild) - if this occurs while walking, stop out of the sun and have a rest and drink. Stay at the closest teahouse.
Lack of appetite or generally feeling bad - common at altitude due to too rapid an ascent.
Painful cough or a dry raspy cough.

In other words anything other than diarrhoea or a sore throat could be altitude sickness. Assume it is, because if you have a headache from dehydration, ascending further is not dangerous, but if it’s due to AMS, the consequences could be very serious. You cannot tell the difference, so caution is the safest course.
Do not try to deceive yourself and accept that you body needs more time to adapt.