"We are getting closer to phase six," said Dr Fukuda, which would mean that a pandemic had occurred. He divided the global outbreak into three types

Dr Keiji Fukuda, WHO's Assistant Director-General, said that the virus had now reached 64 countries with 18,965 laboratory-confirmed cases and 117 deaths.

• countries in North America where virus spread is advanced - Mexico, the United States and Canada;
• the group of countries - which he described as being "in transition", some in Europe, some in Asia, also Australia - where a larger number of cases is occurring, many linked to institutions;
• countries where there is no real evidence of spread into communities and cases are largely travel-related.

Dr Fukuda said that the countries in transition are moving towards community-type spread and include the UK, Spain, Japan, Chile and Australia. "But we are still waiting for evidence of widespread community activity, so that's why we are not in phase six yet".

He went on to say that the WHO was choosing not to describe the outbreak as mild. "This infection can be fatal - in those who have underlying medical conditions, pregnant women and in some people who were otherwise healthy."

And he said that we don't know what will happen in the southern hemisphere in the coming months, or in the northern hemisphere later in the year. The severity of the outbreaks, he said, would vary from location to location, according to the vulnerability of populations and their preparedness.

I asked Dr Fukuda which age groups were most likely to catch the virus and to die from it. "The majority of people who have got infected are under 60 years of age, although some people over 60 have also got infected," he said.

"Those who have got severely ill - with complications such as severe pneumonia and those who have died - have tended to be younger to middle age adults aged 20-40 or so, but not exclusively."

That ties in with the pattern one might expect from a flu pandemic. Normal seasonal flu tends to hit the elderly most severely; in pandemics, it is often young adults who are worst affected.

Also today, two people in Scotland have been admitted to intensive care with swine flu. A 45-year-old man and a 38-year-old woman are being treated in intensive care at the Royal Alexandra Hospital in Paisley. The Scottish government said that they were both in a "critical but stable" condition.

As yet, neither person has any known links to other cases, to travel or to each other. The man is believed to be the first person in Britain to be taken critically ill, suffering only from swine flu. The woman is said to have underlying health problems as does a third patient, a 38-year-old man, who was admitted to intensive care at a hospital in Glasgow last week.