It’s impossible to predict who gets affected by altitude. Altitude sickness is caused by thin air at the high altitudes starting from 3,000 m upward. The highest point of the Manaslu Trek is 5,120 m above the sea level. So, you should give enough time to acclimatize, while trekking in Manaslu region.
The main precaution while trekking is not to go up too high and too fast. If you suffer from symptoms like headaches, shortness of breath, fatigue, nausea, and loss of appetite then descend to a lower elevation immediately and seek medical help.