How healthy is it to get a blood transfusion in thailand?

What for? Getting a blood transfusion is a serious issue and not a do it yourself matter. If you are in the US why do you want to get a transfusion in Thailand? It would be prudent to talk to your doctor about your health issue, if you have one.
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.