In many cases, the illness starts with fever. Sometimes it goes hand in hand with a cough, and perhaps shortness of breath. But for people with a mild case of COVID-19, the illness caused by the new coronavirus, those could be all the symptoms, says Mike Wadman, an emergency physician and co-medical director of the National Quarantine Unit at the University of Nebraska Medical Center in Omaha. That's where 15 people who were aboard the Diamond Princess cruise ship were treated or under observation for the disease when they returned to the United States in late February. 

In addition to fever and cough - two of the three hallmarks of COVID-19 - influenza and coronavirus share a similar list of possible symptoms, including sore throat, headache, muscle aches, fatigue, body aches and runny nose, according to the Centers for Disease Control and Prevention (CDC). People with COVID-19 may experience nausea or vomiting and diarrhoea.

Some warning signs require immediate medical attention. Chest pain usually warrants a visit to the emergency room, no matter what the illness, says Wadman. So does sudden or progressive shortness of breath, or any change in mental functioning, including the inability to wake up or stay awake. The CDC also notes that "bluish-coloured lips or face" is a symptom of COVID-19 that requires immediate medical attention. Older adults and people with underlying illnesses are at increased risk of developing serious illness from COVID-19 that may require hospitalisation.