Hundreds of health officials from across Britain have taken part in an exercise to see how to deal with a flu pandemic.
It is feared that a flu pandemic could kill 400,000 people in the UK
Officials stayed in a bunker at the Department of Health, in London, as part of the logistical exercise.
The strategic trial is thought to be the biggest emergency planning event since the end of the Cold War.
Officials estimate that a flu pandemic would affect one in four people and cause 400,000 deaths in the UK.
It is feared that the H5N1 strain of bird flu, which has killed 167 people, mostly in South East Asia, could mutate into a form that would put millions of people at risk worldwide.
At present, this virus cannot pass easily from human to human, and remains essentially a disease of birds.
But in theory if it mixed with a human flu virus it could mutate into a much more dangerous form.
In the trial scenario, the first infected person was a businessman from Surrey who recently returned from South East Asia.
The operation was an attempt to estimate the way in which certain aspects of the country's infrastructure, such as health, transport, education and food distribution would keep working during an outbreak.
Under current planning for an outbreak, schools would be closed in areas suffering an outbreak, to protect children and stem the spread of the disease.
A dedicated "flu line" would field calls from the public, and be used to distribute the stockpiles of Tamiflu antivirals to those who had symptoms of the virus, although there are concerns the "worried well" may attempt to gain the drugs unnecessarily.
Prototype vaccines could be given to health care workers who are most at risk of catching the flu, but there are currently no plans to vaccinate ministers and government officials.
Hospitals may be put under extreme pressure - with some A&E units forced to closed, and patients transferred to other hospitals.
Employers might be asked to stagger working hours to prevent overcrowding on public transport in a bid to limit transmission.
People with flu symptoms would be asked to stay at home, but it is not expected the government would step in to cancel sporting events or other mass gatherings.
Sir Liam Donaldson, Chief Medical Officer for England, said: "When a flu pandemic hits the country the top priority for the government is to protect the public.
"The World Health Organization has said that the UK is at the forefront of preparations internationally, but it is always necessary to test our responses and improve them where required.
"This exercise is another part of the continual testing, refining, and developing of our plans.
"The NHS is ready to implement its well-rehearsed plan."