Experts have been warning for years that another worldwide flu pandemic is inevitable. The latest outbreaks of “swine flu” are a stark reminder that a deadly virus could emerge in animals, birds or human beings almost anywhere.

The H1 virus in Mexico appears to show signs of spreading quickly between humans through contact as brief as a handshake. As nearly 1.4 million people travelled between Britain and Central America last year, we can probably expect this strain to come to the UK.

H1N1 flu strains similar to the one identified in Mexico circulate seasonally, and most people have some residual immunity to them. But in a process known as antigenic drift, strains of the flu virus undergo genetic changes that make it less familiar to the human immune system, even unrecognisable.

Eight genes have been identified in the Mexican strain — six from swine flu viruses circulating in the US, and two from swine flu viruses from Europe and Asia. The US swine flu virus genes in this new virus are mixtures of swine flu, bird flu and human flu, a combination that generally occurs when viruses of different origin infect the same animal or human and can exchange genetic information.

Predictions suggest that from the time a new virus strain evolves it takes less than a month to spread to about 1,000 cases in its country of origin, and could reach Britain within another two weeks. Once in Britain, it is likely to spread to all main cities within a fortnight.

The risk for the UK (and the northern hemisphere in general) from this Mexican strain will be lower than in other countries because we are coming out of the flu season. It is possible, however, that this strain could come back to affect the UK next winter.

It is worrying that the new virus is affecting young, healthy adults — the group affected by the 1918 flu pandemic.

However, it appears so far that only severe cases are being picked up — what we don’t know is the background of mild illnesses that are not being reported or identified. It is also a good sign that the virus appears to be vulnerable to antiviral drugs that are already stockpiled in Britain.

Avian flu and now this swine flu are currently rated level three out of the WHO’s six levels of pandemic alert — with six the highest. The WHO’s experts meet tomorrow to review the situation.

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