The viral illness, when symptomatic, is characterized by obstruction of the lung pathways (the bronchi) with thick mucous that thickens and may obstruct the airways. With obstructed airways, you get poor ability to breath in oxygen and exhale carbon dioxide, and when severe results in asphyxiation with subsequent cardiac failure and death in those 2%-4% who progress.
In order to treat these lung problems, one must open the airways to give medications that could help this problem. However, this may take a few days, and may require support in the ICU with respirators. Acute respiratory distress syndrome (ARDS) may occur, which is a very severe pulmonary condition with a very high death rate, resulting in worsening inflammation to the lungs and a poorer prognosis.