More than half of the Dutch population is overweight or obese, which is associated with an increased risk of COVID-19 and worse outcomes during a COVID-19 infection. In addition, malnutrition and loss of muscle mass are often reported with COVID-19, as well as with cancer and other diseases. Malnutrition and overweight or obesity can coexist, even within the same individual. It is still unclear which mechanisms contribute to the worse outcomes of COVID-19 and other diseases in case of overweight, malnutrition, and a combination of both. The aim of this study is to identify which parameters are associated with worse disease outcomes through literature research and database research (COVID-19 and cancer, more than 150.000 participants). In addition, a clinical study will be conducted aiming to develop a simple screening tool for the recognition of the coexistence of malnutrition and overweight or obesity in clinical practice.