Obesity, hypertension, diabetes, and specific ethnicities (Black and Hispanic) have been reported to be common comorbidities and possible risk factors for the severity of both COVID-19 and H1N1 influenza infections. Thus, it is important to understand why these four risk factors are common to both COVID-19 and H1N1 influenza infections, and whether a common mechanism exists. Respiratory failure is the most important pathology that contributes to the severity of both COVID-19 and H1N1 influenza infections. Additionally, obesity has been reported to be a risk factor for the development of acute respiratory distress syndrome (ARDS), which is a serious clinical manifestation of both COVID-19 and H1N1 infections. Obesity is a risk factor for hypertension. Most of the previous studies showing hypertension as a risk factor for the severity of COVID-19 and H1N1 infections were either not based on multiple logistic regression analyses or did not include obesity or BMI as an explanatory variable in their multiple logistic regression models. Moreover, similar attention is needed when specifying patients with diabetes or of specific ethnicities (Black and Hispanic) as potentially more vulnerable to either infection, because obesity also correlates with diabetes, and is more prevalent in these ethnicities. Notably, a retrospective cohort study has shown that obesity or high BMI are predictive risk factors for severe COVID-19 outcomes, independent of age, diabetes, and hypertension. Associations between hypertension, diabetes, ethnicities and severity of COVID-19 and H1N1 infections may be confounded by obesity to a considerable extent.