Scientists have discovered that millions of viruses are travelling around the Earth's atmosphere.
The viruses are transported up on the back of organic particles suspended in air and gas – and then rain down upon the ground.
This could explain how genetically identical viruses can be found enormous distances apart – by hitching a ride on smaller particles swept up into the atmosphere.
Scientists from universities in Canada, Spain and the US have also, for the first time, quantified the number of viruses that are being swept up from the ground into the atmosphere.
The number is huge, stretching into the billions per square metre.
While the viruses and bacteria aren't reaching the heights of the stratosphere, they are being lifted way above the zone in which weather conditions are taking place.
The scientists say that at this height, in the troposphere, they can be carried thousands of kilometres before being dumped back down on the Earth's surface.
"Every day, more than 800 million viruses are deposited per square metre above the planetary boundary layer," said University of British Columbia virologist Curtis Suttle, referencing the lowest part of the atmosphere.
This could explain why genetically similar viruses can be discovered in very different environments around the globe.
Living diseases can be swept up into the atmosphere on the back of small particles from dusty soil and sea spray, according to Professor Suttle and his colleagues from the University of Granada and San Diego State University.
Billions of viruses and tens of millions of bacteria per square metre were detected being transported by the atmosphere on platform sites high in the Sierra Nevada mountains in Spain.
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"Bacteria and viruses are typically deposited back to Earth via rain events and Saharan dust intrusions," said study author and microbial ecologist Dr Isabel Reche, from the University of Granada.
"However, the rain was less efficient removing viruses from the atmosphere," Dr Reche added.