ACE2 and viral infectious disease: The increased risk of severe disease and adverse outcome in patients with COVID-19 and co-existing COPD can be attributed to reduced lung reserve, increased ACE2 expression in the bronchial epithelium, chronic lung inflammation, chronic hypoxemia, destruction of the lung parenchyma, expiratory flow limitation, acute exacerbation by viral infection, mucus hypersecretion, and pulmonary hypertension [19].