 The World Health Organization reports that the clinical picture in severe cases is strikingly different from the disease pattern seen during epidemics of seasonal influenza . While people with certain underlying medical conditions are known to be at increased risk , many severe cases occur in previously healthy people . In severe cases , patients generally begin to deteriorate around three to five days after symptom onset . Deterioration is rapid , with many patients progressing to respiratory failure within 24 hours , requiring immediate admission to an intensive care unit . Upon admission , most patients need immediate respiratory support with mechanical ventilation . 
