Acute mountain sickness or AMS is caused due to lack of oxygen in the air at higher altitudes. Headache, nausea, disorientation & breathlessness are symptoms of AMS. AMS can affect anyone regardless of age or fitness. The only cure for AMS is to descend to a lower altitude or to have oxygen from a cylinder.
There are various ways to try and prevent AMS. The best advice we give is to rest in Manali for 2 days to acclimatize to a higher altitude. Shanag Village,, Manali is at an altitude of 2200 meters which is a good altitude to acclimatize to before going further up to the higher Himalayas. Make sure you are well rested and have enough warm clothes. Drink a lot of water as the climate is extremely dry and could lead to dehydration which inturn could lead to AMS. Avoid consuming alcohol and smoking. Avoid exertion at high altitudes. There is medication like Diamox which may lower your chances of getting AMS, however its has side effects such as numbing of fingers and toes and tingling sensations.