Spanish health authorities are on high alert after detecting a rare case of suspected person-to-person transmission of swine flu in Catalonia.
The case has been reported to the World Health Organization - raising the spectre of the 2009 swine flu pandemic, when a similar virus spread rapidly around the world.
The new infection involves the A(H1N1)v variant - a strain normally associated with pigs rather than humans.
Officials said the infected individual, who has since recovered, did not develop flu-like symptoms, and tests on close contacts found no evidence of further spread.
However, the case has unsettled experts because the patient had no contact with pigs or pig farms, according to reports in El País.
That has led scientists to conclude the virus was most likely passed on by another human - a rare event that is closely monitored by global health agencies.
Catalan health officials stressed the risk to the public remains 'very low', and said there is no sign of sustained transmission.
Reuters news agency approached the WHO but it did not immediately comment.
The alert comes against the backdrop of heightened global surveillance. In 2023, the Netherlands reported a human infection with the same swine flu variant in an adult with no occupational exposure to animals.
The 2009 swine flu (H1N1) pandemic indeed began with sporadic, isolated cases in early 2009 before rapidly escalating into a global outbreak by June of that year.
The virus was a new strain of influenza A (H1N1) that resulted from a unique combination of influenza genes, emerging after initially circulating in pigs and eventually passing to humans.
The combination allowed the virus to spread efficiently between people who had little or no immunity.
Although first officially recognized in April 2009, studies suggest the virus was present months earlier, with the earliest known case traced to a five-year-old boy in La Gloria, Veracruz, Mexico, on March 9, 2009.
In mid-April 2009, US laboratories identified the new strain in two children in California, neither of whom had contact with pigs, raising immediate concerns about human-to-human transmission.
Within months, the virus had spread with 'unprecedented speed' across the globe, facilitated by air travel, infecting millions and forcing the roll out of emergency vaccination programmes.
The pandemic predominantly affected children and young adults, likely because older populations had some pre-existing immunity to similar H1N1 strains.
Health authorities say there is no evidence this case represents such a scenario, but stress that early detection and reporting are critical lessons learned from 2009.