At least a third of people who are infected do not develop noticeable symptoms. [11][12] Of those who develop symptoms noticeable enough to be classified as patients, most (81%) develop mild to moderate symptoms (up to mild pneumonia), while 14% develop severe symptoms (dyspnea, hypoxia, or more than 50% lung involvement on imaging), and 5% develop critical symptoms (respiratory failure, shock, or multiorgan dysfunction). People remain contagious for up to 20 days and can spread the virus even if they do not develop symptoms.