Image source, Reuters

News of the US president's Covid-19 diagnosis - and the uncertainty it brings both to the Trump administration and next month's election - has sent a shiver of concern across the global financial markets.

In Asia, Japan’s Nikkei share index fell 0.7%, reversing earlier gains. In Europe, major indexes in London, Frankfurt and Paris all showed falls in early trading, although there was no sign of panic.

Analysts are expecting a stronger reaction when US markets begin trading later. As we reported earlier, the three main indexes - the Dow Jones, the Nasdaq and the S&P 500 - are all expected to open sharply down.

The BBC's business correspondent Theo Leggett says concern about the president's heath is expected to overshadow other factors which would normally influence market sentiment, including continued uncertainty over a $2.2tn (£1.7tn) stimulus plan and what are expected to be encouraging US employment figures.

Image source, PA Media

Another nine Covid-related deaths were registered in Northern Ireland in the week to 25 September, taking the total above 900, official figures show. It is one more than the previous week, according to the latest bulletin from the NI Statistics and Research Agency (Nisra).

The agency counts deaths where the virus is mentioned on the death certificate, and reports 901 deaths up to last Friday. The Department of Health's daily figure for the same date was 578 - more than 300 lower.

The figures from Nisra show 483 deaths in hospital, including the deaths of 81 people normally resident in care homes.

We have more about the situation in Northern Ireland here.

Image source, Reuters

The economic fallout of the pandemic continues. US airlines have begun laying off thousands of workers after efforts to negotiate a new economic relief plan in Congress stalled.

American Airlines will shed 19,000 workers and United Airlines 13,000, although they say they are ready to reverse the decisions if more financing is found.

The airlines have received billions of dollars from the federal government under the Coronavirus Aid, Relief, and Economic Security Act (Cares Act), which was passed by Congress earlier this year. The funding was conditional on the carriers not laying off workers until 1 October.

Airlines worldwide have been hit by a massive fall in demand caused by the pandemic. But other industries have also announced major cuts recently. Earlier this week, Walt Disney said it would lay off 28,000 employees, mostly at its US theme parks.

MP Margaret Ferrier - who travelled by train from London to Scotland after testing positive for Covid-19 - should resign, First Minister Nicola Sturgeon has said.

In a tweet, the Scottish National Party leader said she could not force Ms Ferrier to step down as a member of Parliament but hoped "she will do the right thing".

Former SNP MP Ferrier had the whip withdrawn yesterday after admitting what she had done.

Image source, Reuters

The director-general of the World Health Organization (WHO), Tedros Adhanom Ghebreyesus, has tweeted his "best wishes", external to President Trump and the first lady for a "full and speedy recovery".

Trump, you may remember, has repeatedly criticised the WHO for its response to the pandemic. Earlier this year, he announced he was ending the country’s relationship with the WHO, claiming it was under the "total control" of China, where the virus was first detected.

The US has been the single largest financial contributor to the UN global heath agency, which helps countries promote healthcare and tackle outbreaks.

Dr Tedros defended the WHO's independence at the time of Trump's comments, but promised a review of its response to the pandemic.

Some news from the UK in the meantime. London's transport network will shut down in a "doomsday scenario" without a second bailout, Transport for London (TfL) has warned.

The government agreed to a £1.6bn bailout in May, to keep services running after TfL's income fell by 90% during the coronavirus pandemic.

The deal is due to expire in two weeks. TfL bosses say the network needs £2bn to run until the end of the year.

The government has promised a letter setting out terms for a fresh bailout.

TfL Commissioner Andy Byford said finances were "right on the wire". The transport authority wants almost £3bn to stay afloat through 2021, according to the Local Democracy Report Service.

Image source, Getty Images

President Trump's diagnosis comes after a busy week, running his administration and campaigning ahead of the 3 November election, during which he interacted with many high-level officials.

On Monday, he held a news briefing - giving an update on his administration's coronavirus testing strategy - in the White House Rose Garden which was attended by Vice-President Mike Pence, Health Secretary Alex Azar and Education Secretary Betsy DeVos among others.

At the high-profile TV debate in Cleveland on Tuesday, the president not only shared a stage with his Democrat rival Joe Biden but he was also joined by his most senior aides, including White House Chief of staff Mark Meadows, campaign strategist Jason Miller and policy adviser Stephen Miller.

All his children - and their partners - were also with him. Observers noted that none wore masks during the debate.

During the week, his Supreme Court nominee - Judge Amy Coney Barrett - also visited the White House, and Trump attended both a rally and private fundraiser in Minnesota, the New York Times reports.

It takes five days on average from the moment you are infected to start showing the symptoms, but the World Health Organization says it can take up to 14 days. Incubation period, however, is five days on average.

A loss of a sense of smell may be a more reliable indicator of Covid-19 than a cough or fever, new research suggests.

A study by University College London (UCL) of 590 people who lost their sense of smell or taste earlier in the year found 80% had coronavirus antibodies. Of those people with antibodies, 40% had no other symptoms.

The research only looked at people with mild symptoms, however.

It is thought loss of smell happens with Covid-19 because the virus invades the cells found at the back of the nose, throat and on the tongue. This is distinct from the experience of having a cold where smell and taste might be altered because a person's airways are blocked.

You can read more about the UCL study here.

The dramatic news broke overnight in America, and media outlets have been scrambling to react to what CBS News described as "an earth-shattering announcement mere weeks from election day", external.

For the New York Times, the president's test result came "after he spent months playing down the severity of the outbreak that has killed more than 207,000 in the United States and hours after insisting that "the end of the pandemic is in sight". The paper says, external the test "could pose immediate difficulties for the future of his [re-election] campaign".

It adds: "Even if Trump, 74, remains asymptomatic, he will have to withdraw from the campaign trail and stay isolated in the White House for an unknown period of time. If he becomes sick, it could raise questions about whether he should remain on the ballot at all."

Fox News quotes a source as saying Trump and the first lady are "doing fine", external and are "in the quarantine process" at the White House. The network's medical expert Marc Siegel said his sources had described the pair as "absolutely asymptomatic".

The Wall Street Journal noted a fall in US stock futures and said "the diagnosis throws up a host of uncertainties, external for markets to process", including the question "will the US government be able to function normally?".

Meanwhile, the Washington Post dropped its paywall to allow people to read for free its live updates on the situation, external. In an article on world reaction to the news "despite objections to his handling of the pandemic" the paper noted that "Trump has faced widespread criticism from abroad over his response" and that "foreign researchers have repeatedly criticized the Trump administration for failing to adequately take into account scientific advice".

Politico described Trump as "the world’s highest-profile patient, external of a disease that has killed more than 1 million people".

"A person familiar with the situation said the president was not showing symptoms yet on Thursday," Politico said. "Still, Vice President Mike Pence may need to step in for some tasks if Trump is confined to the White House grounds," it quoted the source as saying.

Image source, Getty Images

Back to news from the US, where Amazon has said more than 19,816 of its frontline workers have contracted Covid-19 since March. The number equates to 1.44% of its 1.37 million workers across Amazon and its subsidiary Whole Foods.

Amazon had faced criticism from employees, unions and elected officials, who have accused the company of putting employees' health at risk. But the online retailing giant said its infection rate was lower than expected.

Amazon has kept its facilities open throughout the pandemic to meet a surge in demand from shoppers stuck at home. Staying open has proven very lucrative for the e-commerce firm, and has added to the wealth of founder Jeff Bezos, who is the world's richest man.

The tech giant's sales soared 40% to $88.9bn (£67.9bn) in the three months ending in June, and its quarterly profit of $5.2bn (£4bn) was its biggest since the company started in 1994.

Meanwhile, some good news for New Zealanders who are planning to travel to Australia. They are to be granted access to the country in the first opening of international borders by either nation since Covid restrictions were imposed.

People will be able to fly from New Zealand to New South Wales and the Northern Territory - and avoid mandatory quarantine - from 16 October.

The nations closed their borders in March in a bid to stop the spread of coronavirus. Officials say the risks are now low enough to justify a "travel bubble".

"The establishment of a travel zone between Australia and New Zealand has been finalised," said Australian Deputy Prime Minister Michael McCormack.

"This is the first stage in what we hope to see as a trans-Tasman bubble between the two countries, stopping not just at that state and that territory."

At first, travel will be limited to New Zealanders.

McCormack said a decision on when Australians may be able to visit New Zealand would be up to Prime Minister Jacinda Ardern.

Image source, Downing Street

The news that Donald Trump has tested positive for Covid-19 may concern US voters - but they will not be the first electorate to see their leader fall ill with the virus.

Among the other world leaders to have tested positive are:

Boris Johnson - The UK's prime minister spent three nights in intensive care in April after contracting the virus, and - after being released from hospital - admitted it "could have gone either way".

Juan Orlando Hernández - The Honduran president revealed in a televised address in June that he, his wife and two of his two aides had tested positive for coronavirus.

Jair Bolsonaro - Brazil's president tested positive in July, after previously testing negative on three occasions. He repeatedly played down risks of what he called the "little flu".

Alejandro Giammattei - Guatemala's president tested positive in September after taking his sixth test. He described his symptoms as being "mild", saying that he had some "body aches" and a "bit of a cough".

Jon Sopel
BBC North America Editor

The tweet from President Trump that he and First Lady Melania had tested positive for coronavirus is simultaneously astonishing - given the security around the president, and the level of testing - and unsurprising, given the ambivalent attitude towards face masks in the White House.

The president's close aide and confidante, Hope Hicks, was diagnosed with the virus on Wednesday. She had boarded Marine One with four other senior staff and the president. None was wearing a mask

She had been with Donald Trump all week - now he has contracted it, and a number of questions arise. How long will he have to quarantine for? Will the next presidential debate in Florida happen? But what if he becomes sick?

The president is 74 and obese, according to the data released a year or so back by the White House medical office. The president's physician has issued a statement saying that he expects him to be able to continue his duties uninterrupted while recuperating, and will quarantine in the White House residence.

There will be some anxiety in Joe Biden's camp as well - after all, the two men were sharing a stage together in a very shouty debate on Tuesday night.

International reaction to the news that the US president and his wife have tested positive for coronavirus has started coming in.

UK Prime Minister Boris Johnson tweeted that he hoped the couple "both have a speedy recovery". Trump tweeted support to Johnson when he himself was in hospital with Covid in April.

Indian Prime Minister Narendra Modi, who received Trump in an on official visit earlier this year, said he wished the first couple "a quick recovery and good health".

EU chief Charles Michel also wished them a "speedy recovery", adding: "Covid-19 is a battle we all continue to fight. Everyday. No matter where we live."

The spokesman for the French government, meanwhile, said the president's diagonisis "demonstrates that the virus spares no-one, including those who have shown scepticism. I wish him a swift recovery."

The president's Democratic challenger in November's election, Joe Biden, has not yet reacted to the news. He was on stage with the president during Tuesday night's presidential debate.

Image source, EPA

We'll have more news and reaction from the US in a moment but first here's a round-up of the latest news across Europe:

• Moscow Mayor Sergei Sobyanin said last night the Covid situation was close to critical in the Russian capital and ordered at least 30% of workers to work from home from Monday. Now Deputy Mayor Vladimir Efimov says that number should gradually rise to half. Moscow reported 2,424 new cases on Thursday and top medic Denis Protsenko says Muscovites have not been wearing masks and have started hugging and shaking hands again.
• Czechs vote in regional and senate elections today, ahead of a state of emergency from Monday affecting schools, leisure and cultural events. A record 3,493 infections and 12 more deaths have been reported in the past 24 hours.
• Germany has reported 2,673 new infections - the highest number since mid-April. Eight more people have died.
• Top French chef Philippe Etchebest wants catering workers to "make a noise" outside hotels, cafes and restaurants at 11:45 (09:45GMT) this morning - after the government warned it could decide on Monday to shut them in Paris, Bordeaux, Nice, Lyon and other cities. France has seen 70,458 new cases in the past week.
• Madrid leaders will appeal to the National Court against an order to limit movement across the Spanish capital from this weekend. They say they will comply with the order. Spain reported another 182 deaths late on Thursday and 9,419 infections.

James Gallagher
Health and science correspondent

It is dangerous to speculate about how the virus will affect any individual person. There are clear risk factors - the older you are the greater the risk of a coronavirus infection becoming life-threatening.

People aged 64-74 with Covid in the US are five times more likely to need hospital care and 90 times more likely to die than somebody in their twenties., external

The US Centers for Disease Control says eight out of 10 Covid deaths in the country have been in the over 65s. The virus also seems to hit men and people who are overweight harder.

But that is the big picture - a pattern seen across the whole population - it is not a way of saying this is what will happen to the US president.

The health of any two 74 year-olds can be wildly different and other medical conditions massively alter how big a threat this virus poses.

We do not yet know if Donald Trump has even developed symptoms.

Meanwhile, the leader of the Scottish National Party in Westminster has called on Margaret Ferrier MP - who travelled on public transport after testing positive for coronavirus - to "reflect very carefully on whether she can continue as a member of Parliament for her constituents".

Ian Blackford told BBC Breakfast that "nobody is above the law, nobody is above the regulations."

He added: "I'm calling on Margaret to do the right thing. It's just a pity that Boris Johnson never did that with Dominic Cummings."

The SNP withdrew the whip from Ferrier on Thursday, after she put out a statement saying she had travelled to Parliament while waiting for the results of a Covid-19 test, and then travelled back to Scotland on public transport despite having received a positive test result.

Pressed if she should resign, he said: "Well I think I've been quite clear in the interview that I've had with you - I've laid it out that she's broken the rules, she's broken the law. She's let herself down, she's let everybody else down.

"I think it's pretty obvious that she has to do the right thing."

Image source, Getty Images

As we mentioned earlier, the US has the world's highest number of Covid-related deaths. But President Trump insists he responded decisively and correctly. So here's a look back at a few of his statements.

22 January (first case confirmed in US) - "We have it under control", external

"We have it totally under control. It's one person coming in from China. We have it under control. It's going to be just fine."

9 March - Compares coronavirus to flu, external

"So last year 37,000 Americans died from the common flu. It averages between 27,000 and 70,000 per year. Nothing is shut down, life & the economy go on... Think about that!"

31 March - "This is not the flu" , external

"It's not the flu. It's vicious. When you send a friend to the hospital... he says goodbye, sort of a tough guy, little older, little heavier than he'd like to be, frankly. And you call up the next day, 'how's he doing?' And he's in a coma? This is not the flu."

3 April - "Choosing not to wear a mask", external

“With the masks, it’s going to be really a voluntary thing. You can do it, you don't have to do it. I'm choosing not to do it, but some people may want to do it and that's OK."

12 July - Wearing masks "a great thing"

"I think when you're in a hospital, especially in that particular setting, where you're talking to a lot of soldiers and people that, in some cases, just got off the operating tables, I think it's a great thing to wear a mask."

21 July - "Wear a mask", external

"We're asking everybody that when you are not able to socially distance, wear a mask, get a mask."

10 September - America “rounding the final turn” of the pandemic, external

“We're rounding the final turn, and a lot of good things are happening with vaccines and with therapeutics."

Unsurprisingly coronavirus has become one of the main topics in the race for the White House. Our Reality Check team has been fact checking some of Trump's comments, as well as those of his Democrat challenger Joe Biden. You can read more here.

Image source, EPA

News of the president’s Covid-19 diagnosis, just a month before the election, has rattled the US stock markets, which are expected to open sharply lower when trading begins on Friday.

Stock market futures show that all three of America’s main indexes - the Dow Jones, the S&P and the Nasdaq - are set to drop by at least 1.5% each.

The Dow Jones could drop by nearly 500 points, the technology-heavy Nasdaq is set to tumble by 2% while the S&P 500 is on course to fall 1.7%.

"It is prime time now for [the] US election," said Jingyi Pan, a senior market strategist at IG Group. "Should the president be absent during this period, that could mean even more complications with the election."

President Trump has long faced criticism for his approach to the coronavirus. In the early stages of the pandemic, he was accused of playing down the severity of the crisis. He has at times suggested preventative measures not recommended by medical experts and ridiculed the wearing of face masks.

The US has the world’s highest number of Covid-related deaths - nearly 208,000. It has also recorded more than seven million cases, about a fifth of the world's total.

Hospital admissions and deaths have declined, however, with the seven-day average for deaths remaining below 1,000 since the end of August.

You can read more from our data team here.