Asymptomatic persons diagnosed with the coronavirus disease are more common than was previously thought – research says potentially as many as 55% to 70% of infections – and they can go on to infect others without anyone ever finding out, The Straits Times reports.

This is one of three reasons that make Covid-19 such a challenging disease to tackle, National Centre for Infectious Diseases executive director Leo Yee Sin has said.

Another reason is that Covid-19 manifests itself as an innocuous and mild illness, but has high virus secretion at its onset.

"This causes infected individuals to misjudge the seriousness of their illness, and have a tendency to continue routine activities without realizing the danger of spreading the infection," she said.

While it was previously unclear if asymptomatic spread is an issue, this has changed as scientists learn more about the virus. Newer findings now suggest that those who do not display symptoms of the disease could be spreading it widely without knowing it.

The number of confirmed cases has surpassed 1.9 million worldwide, according to Johns Hopkins University, which is tracking figures from the World Health Organization and additional sources.

More than 115,000 people have died from the novel coronavirus globally, as more than 453,000 are said to have recovered.