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?
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.
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.
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.
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.