Video report by ITV News Health Correspondent Emily Morgan
The first case of coronavirus has been diagnosed in Northern Ireland, as two further patients have been diagnosed in England.
It brings the total number of UK Covid-19 cases to 16 and experts have warned of the "social cost" if the virus intensifies.
Chief Medical Officer Dr Michael McBride told reporters the patient had come from northern Italy via Dublin.
The news was announced at a briefing in Belfast by Northern Ireland's Public Health Agency.
The agency said it was "working rapidly" to identify anyone the patient came into contact with to prevent a further spread, but they would not confirm where their patient was being held.
Asked whether the person is from Northern Ireland, Dr McBride said: "I think the most important thing here is that we respect patient confidentiality so I'm not going to disclose any personal details about this individual.
"It would be inappropriate for me to do so. It would be wrong for me to do so."
Meanwhile the two patients in England have been transferred to specialist NHS infection centres after they contracted the disease in Italy and Tenerife, according to England's chief medical officer Professor Chris Whitty.
One patient was taken to the specialist infectious diseases centre at the Royal Liverpool Hospital and the other to the Royal Free Hospital in London.
Brits are among holidaymakers on lockdown at the H10 Costa Adeje Palace in Tenerife, after two Italian tourists tested positive for the virus during their stay.
Around 50 Brits are among 130 guests now able to leave the hotel complex, as they are understood to have arrived on Monday, after the guests who were diagnosed had already left.
One of the cases in England is understood to be a parent from Burbage Primary School in Buxton, Derbyshire, who caught the virus while in Tenerife.
Parents were told the school would be closed on Thursday due to a "confirmed case of coronavirus amongst our parent population".
The school, which has 350 pupils, sent a message to parents via WhatsApp on Wednesday night saying the decision had been taken as a "precautionary measure and to enable a deep clean to be completed".
Dr Fu-Meng Khaw, centre director for Public Health England (PHE) East Midlands, said PHE was contacting people who had close contact with the patient.
He added: “Close contacts will be given health advice about symptoms and emergency contact details to use if they become unwell in the 14 days after contact with the confirmed cases."
Buxton Medical Practice in Derbyshire, a two-minute drive from the school, urged patients not to attend on Thursday due to a confirmed case of coronavirus.
England's chief medical officer, Professor Chris Whitty, warned that onward transmission between people in the UK was "just a matter of time in my view".
Speaking at a Nuffield Trust summit, he said: "If this becomes a global epidemic then the UK will get it, and if it does not become a global epidemic, the UK is perfectly capable of containing and getting rid of individual cases leading to onward transmission."
But he said onward transmission was likely, adding: "If it is something which is containable, the UK can contain it. If it is not containable, it will be non-containable everywhere and then it is coming our way."
He said there could be a potential "social cost" if the virus intensifies, which could include reducing mass gatherings and closing schools.
"One of the things that's really clear with this virus, much more so than flu, is that anything we do we're going to have to do for quite a long period of time, probably more than two months," he said.
"The implications of that are non-trivial, so we need to think that through carefully.
"This is something we face as really quite a serious problem for society potentially if this goes out of control."
So far in the UK, 7,690 people have been tested for the virus and of the 16 to have tested positive, eight have so far been discharged from hospital.
In China, where the virus originated, 78,497 cases have been reported, including 2,744 deaths.
World Health Organisation (WHO) director-general, Dr Tedros Ghebreyesus, told a press conference in Geneva on Thursday that coronavirus has the potential to become a global pandemic but this stage had not been reached.
Public health advice remains to wash hands with soap, not rub the face and maintain a distance from people who are coughing and sneezing, he said.