Many people complain that going on holiday caused them to catch a cold, and now we may know why!
"Vacation colds" and "holiday influenza" are perennial moans made by holidaymakers who claim to remain steadfastly healthy in the office, but as soon as they fly abroad to hit the beach, immediately succumb to the local circulating lurgy.
The reason, scientists at Cambridge University are suggesting this week, could be because disruption to your body clock makes infecting viruses grow up to ten times faster in your cells.
Writing in PNAS, Rachel Edgar and her colleagues infected mice with the rodent equivalent of glandular fever (EBV), or a strain of influenza virus. Animals that were infected in the morning developed significantly higher levels of circulating virus, and greater spread of the infection.
The same was true using cultured cells, which also have a chemical clock ticking inside them.
To probe how the effect was occurring, the team repeated the experiments using mice lacking a key gene, called Bmal1, which is a crucial cog in the clock mechanism. In the absence of Bmal1, virus replication was high at all times. In cells and mice with normal Bmal1 genes, infection with either of the two viruses also strongly disrupted the Bmal1 gene activity inside the infected cell, effectively causing the clock to tick at the wrong rate.
The purpose of throwing this metabolic spanner in the works is that the purpose of the body clock, which is active inside every cell in the body, is to optimise the activity of the cell to meet metabolic demand, which are low when we're asleep and high when we're awake.
By advancing the clock and revving up cells, an infecting virus renders the cell a much more hospitable environment in which to reproduce rapidly.
So why is this relevant to a passport bearing beach-goer? Because jumping aboard an intercontinental airliner inevitably disrupts the body clock, making the traveler more susceptible to infection and a better viral host.
The Cambridge team also point out in their paper that their results may explain one other observation: that colds are commoner in winter. Relatively shorter days in winter mean that activity of the Bmal1 gene, which guards against infection, is lower, which may explain why 'flu finds it easier to spread through the population in winter...