Travel insurance is a must for everyone traveling to this area. It would help if you did travel insurance before booking the trek.
Travel insurance covers medical and emergency returns. Besides this, Travel insurance also comprises helicopter rescue. As well as clearing expenses at high altitudes (for trekking members).
Giving it to your travel company before you trek would be best. Above the Himalayas would help you to get travel insurance. This will protect you from potential altitude sickness.