Welcome to the inquiry on the BBC World Service.

I'm Tanya Beckett.

Each week, one question, four expert witnesses

and an answer. It was a normal Tuesday in Victoria, Australia's most popular state.

Temperatures had dropped to the low teens after the blistering summer weather and life was returning to normal after the lifting of the coronavirus lockdown some weeks before. But behind the

apparent calm there was a worrying rise in cases in the state's largest city, Melbourne.

Suddenly came the shock announcement that anyone without a permit

would not be able to cross the border into New South Wales after midnight.

Travel between Sydney and Melbourne was all but stopped.

It was the first time that the border had been closed since the Spanish flu pandemic a century ago,

and it was swiftly followed by a return to lockdown in Melbourne.

Was the rise in cases connected to cooler weather?

Countries in the northern hemisphere started to voice more openly a long-held concern.

Will the pandemic get worse in the winter? This week on the

inquiry we're asking will winter bring a second wave of COVID-19.

Part 1, Forest Fires The expectation has been that SARS-Cove 2 will be a wintertime infection if it behaves anything like its sister viruses.

Our first expert witness conducts scientific research into the seasonality of viral transmission.

My name is Michaela Martinez and I'm an assistant professor in the Department of

Environmental Health Sciences at Columbia University.

Michaela says that cases of infectious diseases peak and trough throughout the year.

Every human infectious disease has a seasonality to it.

We have wintertime infections like blue and the common

...