Symptom-free 'super spreaders' carry as much Covid-19 as patients left fighting for their lives, according to new research.
They have the same amount of coronavirus in their noses and throats, say scientists, endangering the lives of those around them.
One-in-five people infected show no signs of the potentially deadly disease, the study revealed.
The pandemic is being fuelled by those who have no idea they have been infected, explained lead author Professor Sung-Han Kim.
He said: "Our data add further support to the general public use of face masks - regardless of the presence of symptoms.
"They suggest the scope of testing should be expanded to include asymptomatic individuals in high-risk settings - such as nursing homes or healthcare facilities."
Vulnerable family members - such as elderly parents or grandparents - will be particularly at risk.
Professor Kim, of Asan Medical Centre, Seoul, said: "People infected with Covid-19, but who don't have symptoms, still carry potentially transmissible virus."
Prof Kim and colleagues compared 213 people, all of whom lived in Daegu City, South Korea, and had tested positive for the infection. This doesn't necessarily indicate the presence of live virus.
They had mild or no symptoms and were admitted to a dedicated care facility for isolation and monitoring. They were mainly in their 20s and 30s.
They were classified as symptomless if they had no fever; chills; muscle pain (myalgia); fatigue; runny nose (rhinorrhea); blocked nose; loss of taste or smell; sore throat; swallowing difficulties; cough; phlegm production; coughing up blood; headache; dizziness; loss of appetite; nausea; vomiting, abdominal pain; and diarrhoea.
Before the isolation period--an average of six days from the first swab test--around a fifth (41; 19%) people didn't developed any symptoms: 39 (95%) of them had a further nose and throat swab test, 13 days, on average, after their initial diagnosis, to measure viral load.
Of the remaining 172 (81%) with mild symptoms, 144 were retested, adding up to a total of 183 who were included in the final analysis.
Over half of those without symptoms (21; 54%) tested positive for Covid-19 as did nearly two thirds of those with mild symptoms (92; 64%).
Prof Kim said: "Further studies are needed to clarify whether the persistence of viral DNA in people without any symptoms warrants precautionary quarantine measures."
He added: "Considering most asymptomatic individuals with Covid-19 are likely to go unnoticed by healthcare workers and continue to reside within communities, such individuals may act as an essential driving force for community spread and the ongoing pandemic state."
Until such time as it becomes clear how long, and to what extent, symptomless people might be infectious, testing should be extended to certain groups as a precautionary measure, recommend the researchers.
Last month a study found children can carry larger amounts of the virus than adults requiring emergency ventilation - yet show no symptoms.
The findings were based on nose, throat and blood samples from almost 200 young paediatric patients in the US.