BMI or the body max index is the basis if your weight is appropriate to your height. Oftentimes, many people do not have the normal BMI because of obesity or being overweight and underweight. So, if you are having a BMI which is below 20, chances are, you are going to experience Iron-deficiency Anemia, wherein the body is not able to get enough iron, and this would result in an imbalanced functioning of your body which would manifest in many ways. Also, having low BMI gives you a higher risk of acquiring lung problems especially those that are infection related problems. Hope this answer helps.