Swine Flu Phases:
Flu is broken down into a number of different types: there's flu A, flu B and flu C. Flu A is the bird form and, of course, humans also have a flu A. Then we further categorise it according to the surface molecules, the H (for Haemagglutinin) and the N on the surface.
Please Note: This does NOT include any possible future mutations of the current strain similar to that which happenen in 1918 with the Spanish Flu (which killed over 50 Million people).
 This new strain is a H1N1 virus, which we know circulates in humans, pigs and birds. It's possible that the pig initially got some of its flu from a bird and then co-mixed that with the human form to produce this hybrid, which is why you can find elements of all three.
Stage 4 shown here represents the possibility of this virus mutating and the potential death rate from a similar mutation to the spanish flu of 1918.
Approx 30% of people infected with the Spanish Flu died.
The main focus now by infection control organisations like the Centres for Disease and Prevention (CDP) and the WHO, will be to interrogate this virus at a molecular level.
In other words, look at the genetic make-up, sequence the virus, and once they've got the genetic sequence you can begin to unpick where it's come from, what its origins are, and therefore begin to build the story as to what its likely outcome will be.
W.H.O.: Swine flu viruses are most commonly of the H1N1 subtype but other subtypes g (H1N2, H3N1 and H3N) are also circulating. The H3N2 swine virus was thought to have been originally introduced into pigs by humans.
An influenza virus containing genes from a number of sources is called a "reassortant" virus. Swine flu viruses are normally species specific and only infect pigs, but sometimes they cross the species barrier to cause disease in humans.
The health protection agency in the UK has drawn up an algorithm as to how they intend to assess people. The first point on the algorithm is geography. Have people come in from an area where we're seeing disease activity? That includes obviously parts of the US now but chiefly Mexico.
It is important to plot these "human migration events" so that better global planning strategies can be implimented for any future outbreaks.
Symptom Algorithm
Then there is a symptom algorithm. It's a temperature of more than 38 degrees, or a history of a temperature of more than 38 degrees and not only respiratory symptoms (for example, a runny nose) or a headache but also diarrhoea and vomiting.
Some of the US cases have presented with diarrhoea, though it's not clear actually if those symptoms were incidental. But because this is a foreign virus getting into people it could present in an atypical way.
They're using that initial screening and then activating various molecular tests, using tests that can detect the genetic material of the virus to see if we're onto this form of flu.
W.H.O.: Swine flu tends to be associated with high morbidity (that is, it infects a large proportion of an area's population) but low mortality (less than 5 per cent of infected patients die of the disease).
Generally clinical symptoms are similar to seasonal influenza but reported clinical presentation ranges broadly from asymptomatic infection to severe pneumonia resulting in death.
Since typical clinical presentation of swine influenza infection in humans resembles seasonal influenza and other acute upper respiratory tract infections, most of the cases have been detected by chance through seasonal influenza surveillance. Mild or asymptomatic cases may have escaped from recognition; therefore the true extent of this disease among humans is unknown.
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