It has not done this yet. We have seen just a few thousand cases in Mexico city with a population of 25 million people.
So it has not spread anywhere near what would be considered to be of pandemic proportions as of yet.
If in the next week to two weeks it starts to spread more widely, we will know we have a pandemic.
Otherwise, it could just fizzle out – it may not be a fit virus to maintain a pandemic.
Another scenario is that it is not a pandemic strain, but is still capable of carrying on and mutating, reinventing itself as a pandemic virus in the next six months or so.
The strain could pick up the ability to improve its fitness to spread.
There is an immediate short-term need to look at this virus, but there is also the possibility that it will retrench and have another go ate becoming a pandemic strain in the future.
Whether this strain is able to mutate is purely down to chance – it is what Charles Darwin spoke about.
There could be random mutations which lead to the right permutation to create a deadly pandemic strain.
Flu has only done this three times in the last 100 years so it is a rare thing to happen, thankfully for us.
There is this possibility of mutation but I think it is too soon to panic.
The virus does not really seem to be taking off that quickly in Mexico.
We should be worried, but not to the extent that we start to panic.
We will still need to be looking for a vaccine and stock-piling antiviral drugs.
We need to make people aware of what they can do to limit their chances of picking up an infection.
Flu is good at adapting and finding ways of getting past our armour.
But it is still a complex organism. It still has to get all the genes lined up and synchronised with each other, doing the right thing, before it is a really fit virus to spread and then become a dangerous pandemic strain.
We hope, in the case of the swine flu currently being seen in Mexico and elsewhere, that it will simply fizzle out.
But there will be a pandemic sooner or later. This might not be it, but it will come – we just do not know when that will be.
Professor Nigel Dimmock is emeritus professor of virology at the University of Warwick.