The number of swine flu cases is rising rapidly and experts are warning in the worst-case scenario 30% - or one in three of the UK population - could become infected and up to 65,000 people could die this winter.
These sound like big, scary numbers, but what do they actually mean and how have they been calculated?
HOW MANY WILL CATCH SWINE FLU?
30% of population might catch swine flu = 18.3m people
15% of those might suffer complications = 2.74m people
2% might need hospital treatment = 366,000
0.1-0.35% fatality rate = 18,300-64,050
In England, the number of people with swine flu symptoms who have consulted their GP is now equivalent to a bad bout of seasonal flu during the winter months - although Wales and Scotland have reported fewer cases.
The Department of Health says figures are expected to continue to rise and will probably peak in the autumn and winter.
Health officials say up to 30% of the population may become infected with swine flu, based on analysis of how this pandemic is spreading, and of those, 15% may suffer from complications and need medical treatment.
Up to 2% may need to be admitted to hospital and 0.1-0.35% may die from the virus - or roughly between one in 1,000 and one in 300 patients will die.
The estimated fatality rate, based on figures from the UK and abroad, suggest between 19,000 and 65,000 people may die from swine flu.
UK FLU DEATHS: HOW THE NUMBERS COMPARE
The government says up to 12,000 people die every year from seasonal flu. The fatality rate is based on the same 0.1-0.35% range - but fewer people will catch flu in an average year which is why the number of deaths is lower.
The estimated 30% swine flu infection rate is based on previous experience of pandemics.
In 1957-8, between 25% and 30% of the population contracted Asian flu and 33,000 people died - a fatality rate of 0.25%. Ten years later, 25 to 30% of the population caught Hong Kong flu. It claimed 30,000 lives, a fatality rate of 0.2%
The worst flu pandemic occurred in 1918, when Spanish flu is estimated to have killed up to 50m people worldwide.
HOW MANY CHILDREN WILL BE AFFECTED?
Children have been identified as one of the high risk categories - Department of Health officials say up to 50%, or 5.9 million, under-16s could become infected with swine flu in the worst-case scenario.
The young are more likely to catch swine flu because their immune systems are less developed, they shed the virus more than adults and pass it on more easily because of their social behaviour.
Although this does not necessarily mean there will be more deaths among the very young, health officials say they may be more likely to suffer complications and need some kind of medical treatment.
HIGH RISK GROUPS
Certain groups, including pregnant women and people with underlying health problems, have also been called at risk groups. But it is impossible to say what the increased risk is above the average because it will vary greatly from person-to-person.
Pregnant women commonly have an increased susceptibility to flu because their immune system is compromised by their pregnancy. Flu symptoms such as a fever can also pose a risk to the unborn baby, unless treated.
But the official advice is that the vast majority of women affected will still suffer only mild symptoms.
People with underlying health conditions, such as chronic lung disease, heart disease and severe asthma, also have an increased susceptibility to flu because their immune system is compromised by their condition and/or the treatment they are receiving.
They are also more likely to develop complications, such as pneumonia.
WHAT ARE THE COSTS TO THE ECONOMY?
The government is also suggesting that up to 12% of the workforce could be off work because of illness during the peak weeks of the pandemic.
Normally workers are absent an average of eight days a year, or once every 30 working days. People who contract swine flu could be absent an additional one to two weeks - or 10 working days.
The situation could be exacerbated by the closure of schools, forcing many parents to stay away from work to look after their children.
This reduction in the workforce could have a major impact on everything from the running of public services to the supermarket supply chain.
Economists have warned if infection rates do reach the worst-case scenario, the UK economy could contract by as much as 7.5% next year - down from this year's forecasted -4.4%.
They say a flu outbreak in the autumn would put additional pressure on already-fragile businesses and could spark a period of deflation.
The government's preferred measure for inflation, the Consumer Prices Index (CPI) could dip below zero to -1% throughout 2010-2012 - postponing hope of an economic recovery for a couple of years.
FLU-LIKE ILLNESS IN ENGLAND, WALES AND NORTHERN IRELAND
The map below shows the concentration of all influenza-like illnesses, including swine flu, across England, Wales and Northern Ireland, by late July, broken up by primary care trusts. The number of new cases for this week in England was estimated at 110,000, a rise of 10,000 over the previous week. Levels of illness have also risen in Wales and Northern Ireland.
Rates of flu-like illness have risen slightly in Scotland during the past week, especially in the east of the country.
Scotland has a different system of recording flu-like illness, choosing to collate consultation rates rather than the number of cases diagnosed. The differing methods mean rates of illness in Scotland and England, Wales and Northern Ireland are not directly comparable on the same map.