Focus on more disciplined routine. It is not always about numbers on weighing scale. If you are thin,  it can be you have that kind of body type. But yes if you are not feeling healthy., low immunity and  so. Here are some advice for gaining healthy weight. 1 . include food combinations like cereals and protein in your diet. Have major meals containing salads, dal., Chapati, vegetable and curd. Note:eat as per your hunger. Enjoy eating. Don't stuff yourself. Include new new healthy recipes in your diet. 2.have a complete breakfast like combinations e. g. poha with milk and a fruit. Add variety and balanced all the nutrients. 3 do take healthy snacks 4.of course add some good exercise routine for staying. Fit