My flu resistance becomes low with intense training. is this normal?

Yes: Get vaccination for flu. What you are getting is common cold, not flu. It is good that you exercise, but do not do it so intensely that your risk hurting yourself. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex, if you have sex.