How Coronaviruses Cause Infection—from Colds to Deadly Pneumonia

The 2019 novel coronavirus (2019-nCoV) behind the ongoing outbreak - which the World Health Organization has declared an international public health emergency - was named after the family of viruses it belongs to. The term “coronavirus” may have initially been unfamiliar to many, but most everyone has encountered milder forms of such viruses, of which four strains cause about a fifth of common cold cases. But until less than two decades ago, all known human varieties caused illness so mild that coronavirus research was something of a backwater. The major difference between coronaviruses that cause a cold and those that cause a severe illness is that the former primarily infect the upper respiratory tract (the nose and throat), whereas the latter thrive in the lower respiratory tract (the lungs) and can lead to pneumonia.