By Nick Triggle
Health reporter, BBC News
A flu pandemic is imminent, experts say
World experts may be saying a flu pandemic is imminent, but on the ground health protection officials in the UK are still battling to stop its spread.
The approach is simple - isolate the virus and kill it before it spreads.
Health Secretary Alan Johnson has called it the "containment stage" - and it is the method that is being adopted in other countries that are seeing swine flu cases.
It basically involves putting a ring around any outbreak and trying to zap it quickly.
In practice, this means when a person is infected they are isolated, as are all the people they came into contact with.
Anti-virals are then given en masse - even to those who are not displaying symptoms - in a bid to ensure it does not spread further.
Experts call this using drugs prophylactically, which basically means using them to prevent the disease developing.
There is sketchy evidence that this actually works, but at worst the drugs will at least lessen the symptoms and shorten the illness if someone does develop flu.
This is the tactic that was taken in Devon when a 12-year-old girl was diagnosed on Wednesday.
She had come in to contact with 50 fellow pupils but the decision was taken to give all 230 pupils in her year anti-virals.
The school has now been closed and local health officials are monitoring developments.
A similar approach was taken for the Scottish couple and the people they had come into contact with. So far none of their friends and relatives have developed the flu.
But there is no guarantee this will work. The World Health Organization has already declared phase five has been reached - one step away from a pandemic.
This means that sustained human-to-human transmission is being seen within two countries - Mexico and the US.
In the UK, all the cases that have been confirmed were in people who have been to Mexico.
Chief Medical Officer Sir Liam Donaldson admits if the UK were to see onward transmission to people who have not visited the affected areas it would suggest "an escalation of the problem".
He said this would also mark a change in the approach with the anti-virals saved for only those showing symptoms of the flu.
The problem the UK is facing in an era of global travel is that, even if its approach works initially, flu defences are only as strong as their weakest link.
Sandra Mounier-Jack, an infectious diseases expert from the London School of Hygiene and Tropical Medicine, said: "Once it starts to spread it is hard to contain.
"The UK has good plans in place but not every country does, so that means even if we slow the spread at this stage, if it gets a foothold elsewhere it gets harder to stop."
It means that while the UK is trying to contain the flu, plans are being put in place for a full-blown epidemic.
Ministers have already discussed restricting travel and public gatherings at their Cobra emergency meetings.
Extra anti-virals and masks have been ordered and a TV, radio and newspaper information advertising campaign started on Thursday.
This will be followed by leaflets which will be delivered to every home in the country next week.
But despite the developments of the last week, experts are still relatively upbeat because of the nature of the flu.
Outside of Mexico it seems to be a relatively mild virus with only one person - a Mexican toddler who had travelled to the US - dying from it.
Sir Liam admitted the flu strain could still mutate into something more deadly.
But he added at the moment most people who get it "will make a good recovery".
"It's a nasty illness, but it's short and they will recover.
"To put things in proportion, in any flu, even the seasonal flu, there are some deaths, often of elderly people and the very frail.
"What we will see is many more cases, but on the whole most people make a good recovery from flu."