In addition, there is presumably no pre-existing immunity in the population against the new coronavirus and everyone in the population is assumed to be susceptible. Clinical presentations of COVID-19 range from no symptoms (asymptomatic) to severe pneumonia; severe disease can lead to death. While the majority of cases (80%) are milder respiratory infections and pneumonias, severe illness and death is more common among the elderly with other chronic underlying conditions, with these risk groups accounting for the majority of severe disease and fatalities to date.