The transfer of swine flu from one person to another outside Mexico marks a "very significant stage" in the progress of the outbreak, an influenza expert said today.
Graeme Pacitti, 24, from Polmont, Falkirk, tested positive for the virus after coming into contact with football team-mate Iain Askham, who fell ill after his honeymoon in Mexico.
Professor John Oxford, professor of virology at Barts and the London NHS Trust said transmission of the disease from one person to another within countries other than Mexico had taken the world a step closer to a full blown pandemic.
"It is a very significant stage upwards. The fact that it has happened in two countries already apart from Mexico is what forced the World Health Organisation to step up the alert level yesterday."
He said that the news of the first case of human-to-human transfer in the UK will make efforts here to control the spread more urgent.
"It has happened in other countries so we knew it was likely to happen here. It just makes it a bit more pressing because it is in our back garden. It is something that one doesn't want to have but we have got it now so we will have to deal with it. The whole thing has been kicked into action now."
A spokesman for the Health Protection Agency said: "It is not a surprise that it has spread in this way but how significant that is going to be is going to become clear over the next few weeks."
Dr Alan McNally, senior lecturer and influenza diagnostics researcher at Nottingham Trent University, said that the vast majority of the UK's possible cases currently being investigated are likely to have originated from contact with other infected people in this country.
The next step will be watching how quickly and easily the infection will spread from person to person, said Professor Oxford.
"We will find out in the next week how transferable it is. So far all we have got is one-to-one transmission not one person to three or four."
The current good weather could also slow the spread.
"It will be quite difficult for the virus to move fast at the moment. You rarely get outbreaks of flu in the summer, it is not completely unknown but it is rare," he said.
He added: "Hygiene is going to be very important now. Everybody is going to have to get on board and help in this outbreak by washing hands, using disinfectant and tissues."
If the virus spreads, said Professor Oxford, it will do so from several points around the country rather than starting from one location.
But he said that Britain was well prepared to tackle swine flu. "I can't think of anywhere I'd rather be at the moment," he added.