coronavirus have no detectable symptoms at all. World Health Organisation figures suggest that while 5% of Covid-19 infections are very serious, with patients requiring a ventilator if they’re to survive, around 80% of cases are mild or asymptomatic and people with the bug can carry on with their lives unaware that anything is wrong.But a new study from the University of Ulsan College of Medicine in South Korea shows that these ‘perfectly healthy’ Covid carriers can be as infectious as more severe cases.The work confirms earlier work done by a team led by Jeffrey Shaman at Columbia University in New York who traced the original outbreak in Wuhan, China.Sung-Han Kim's team at the University of Ulsan took nose and throat swabs from 39 people.