Mountain sickness (altitude sickness) : If you climb very rapidly up during the first days (even though you are in a very good form), and if you do too many efforts in the first days, if you have a cold or an angina at the time of your departure to travel or trek, YOU are a CANDIDATE of THE ALTITUDE SICKNESS!!
(There are many other reasons that one doesn't know but those are sure, in fact some have this and of others dont have...). It can simply appear by headache (aspirin), nausea, but it also can go farther. There is a current technique of prevention (to see with your physician) that has good results on 80% of people who has tried. Before an important altitude travel or trek, take a half tablet of DIAMOX (not more than the half tablet!) during the 2 days before the passage in the high altitude and drink much more, more than that usual, because the Diamox is a diuretic. But mostly not to take some more and certainly not, during all the day since the departure.. If you take it many, you risk a bad dehydration that also can be very dangerous...
There is a second medicine that relieves, but it is also to take exactly on that moment or one has headache, few difficult in breathing, etc, it is a medicine homeopathic COCA 9 or 6 CH. (3 small pills under the tongue and you feel better during 3 - 4 hours). But its useless to take some before, it doesn't serve anything.
And then there is local medicine Nepalese, the garlic soup, soup with a lot of garlic, you eat lot of garlic during hiking days .. Certainly it feels a little, but it is an ancestral medicine that also made its proofs....
In the days before leaving or at the time of departure to the trek, you have a cold or, angina, or pulmonary problem, treat correctly!! (Antibiotic, paracetamol, efferalgan) because it gets worse very quickly while bringing up and encourage also problems of altitude.

Before leaving your country, consult your physician.