The reason I was nervous about this is that there seems to be no pattern as to whom it affects. It does not depend upon the fitness of the trekker or on how many times that person has been to altitudes in the past. This seemingly random process worried me that it would prevent me from fulfilling my ambition of reaching Gokyo Ri and Everest Base Camp.
The best way to avoid the illness, or to reduce your chances of suffering from it, is to ascend gradually building in rest days to the schedule. One group I met was forced to descend after they ascended too quickly and had to abort their attempt for EBC. I was certainly glad that we had taken our ascent slowly and had an acclimitisation day in Namche.
The most common symptom of altitude sickness is a headache. In fact around 4000m it is almost universal to have one. Mine was a very dull and mild headache, not causing any concern for me as I knew to expect one. I would just pop some paracetamol and it cleared overnight. The headache is caused by the body pumping more blood to the head in order to give the brain the oxygen it is used to having at lower altitudes. Therefore deep breathing can help to alleviate it and this perhaps explains why my headache cleared overnight (compounded by the fact that my body became used to the altitude over time).
I also drank plenty of water. As you are exercising in sometimes cold conditions it can slip the mind not do drink. But not only will it help with any headache but it will defend you against the serious problem of dehydration. I aimed to drink at least 3-4 litres a day.
The only other mild symptom you may experience is a loss of appetite. Luckily I was not affected by this and found myself being called upon the finish the others meals – a role I delighted in.
You are rightly advised, if the symptoms progress into a more serious headache, dizziness and nausea, not to ascend any further; if the conditions becomes even more serious you should descend immediately and never alone. It can be a serious threat and so groups should always err on the side of caution. But for the most part it shouldn’t put your trek in jeopardy in any way.