While many people who become infected with coronavirus develop symptoms, others may carry it without realising

Researchers say it’s vital to know how many people are “silent spreaders” to understand the virus

@davidshukmanbbc explains

[Thread] http://bbc.in/CoronavirusSilentSpreaders

It all started in a church in Singapore on 19 January

Amongst the congregation were a couple who had just arrived from China

The couple seemed healthy - displaying no symptoms

At the time, no symptoms meant no chance of spreading it

http://bbc.in/CoronavirusSilentSpreaders

A few days later, the couple fell sick

And over the following week, others experienced symptoms too

Singapore's Ministry of Health were “extremely perplexed” at how people who didn’t know each other were becoming infected, while showing no symptoms

http://bbc.in/CoronavirusSilentSpreaders

Singapore mobilised “disease detectives”

They found many of those who had been infected had been at that same church service

One unexpected finding was the woman who contracted the virus by sitting in the same seats as the couple later that day

http://bbc.in/CoronavirusSilentSpreaders

The church in Singapore uncovered what we now know as "pre-symptomatic transmission"

Studies highlight a critical period of 24-to-48 hours before the symptoms develop when people can be highly infectious

http://bbc.in/CoronavirusSilentSpreaders

The perplexing fact remains that some people never show symptoms - they have asymptomatic infection

Studies suggest a range of possibilities for how many cases are asymptomatic

It's impossible to know how many cases of infection remain hidden from view

http://bbc.in/CoronavirusSilentSpreaders

"If you have people who don't know they're ill while using public transport and health care facilities, inevitably that's going to increase transmission”

In the search for an answer, scientists are pushing for tests for the entire population of one city

http://bbc.in/CoronavirusSilentSpreaders

At the moment, there is no way of telling who among society may be carrying the virus without knowing

That’s why governments are saying it’s essential to adhere to social distancing, cooperate with tracing efforts and self-isolate if necessary

http://bbc.in/CoronavirusSilentSpreaders

To understand more about asymptomatic infection: http://bbc.in/CoronavirusSilentSpreaders