2.3  Variability of clinical presentation
Before the first outbreak of SARS, a limited number of HCoVs such as HCoV-229E were frequently found to infect humans, and were widely circulating in human populations causing only mild illnesses like the common cold [3,25]. However, SARS, MERS and SARS-CoV-2 present with a spectrum of disease severity ranging from flu-like symptoms to acute respiratory distress syndrome [25,26]. Early on, these patients are usually treated with conventional medications which had no clinical benefit, resulting in spread to health care personnel. For those with flu-like symptoms or even more severe disease, it would not be immediately evident that this is an atypical and virulent form of a coronavirus. For comparison, influenza has an estimated mortality rate of 0.07%–0.2%. A high index of suspicion is helpful but not foolproof.