How to improve our immune system so we can't easylly to get sick ?

No sure way.: Staying in good physical shape with regular and moderate exercises plus a healthy diet may be helpful. Make sure your plasma vitamin D3 level is optimal since vitamin D3 is an important immune regulator. I would like to see the 25 oh vitamin D3 level between 60-80 ng instead of 30. More fruits and veggie. Wash your hands often especially before eating. Good sleep habit and mental attitude.