What Causes Abdominal Cramps, Back Pain And Weight Loss?

My grandfather has been sick for about two months now for unknown reasons. Since then, he s lost a great amount of weight because he hasn t been eating right. He s been vomiting profusely from some of the things he s attempted to eat to try and help him feel better. He s been having an intense abdomen pain, along with back pain and weight loss. And no one has been able to figure out what was wrong.