Rain and dust
About 20 years ago, researchers discovered that genetically identical viruses had spread all around the globe. Because viruses are not able to travel independently, but depend on a host to do this (often a bacterial host), their dramatic spread was a mystery for many years. This mystery has now been resolved. The international research team discovered that viruses find it easy to travel through the air, just like bacteria do. However, because the average virus is easily 100 times smaller than a bacterium, they are far more successful. For example, they are able to fit on even smaller dust particles, which enables them to stay in the air longer. The rain and dust storms that bring bacteria back down to the earth's surface also have less of an impact on the far smaller viruses.
Thousands of kilometres
Viruses are swept into the atmosphere by dust clouds and sea spray. They ultimately reach a height of some 6 to 10 kilometres in the troposphere, just under the cruising altitude of passenger aircraft, which is where all of the various weather phenomena manifest themselves. As such, the air here is hardly calm and wind speeds are high. These conditions enable viruses to travel thousands of kilometres before descending to earth again.
Billions of viruses per square metre
Every day, between 260 million and 7 billion viruses descend on every square metre of the earth’s surface. The higher you go, the more viruses you will see swirling downward. The research was carried out on two mountain tops in Spain's Sierra Nevada, where billions of viruses are deposited every day. This makes the number of viruses that descend from the atmosphere onto the surface of the earth 9 to 461 times bigger than the figure applicable for bacteria.