Bacteria get everywhere - even nine miles high among the clouds, a study has found.
Scientists discovered "significant" numbers of living bugs in the middle and upper troposphere, the airy layer five to nine miles above the Earth's surface.
The microbes could have a previously unrecognised impact on cloud formation, according to the research.
Long distance travel by the airborne organisms may also help spread infections around the world.
The bugs were discovered in air samples scooped up by a DC-8 aircraft flying over both land and sea across the US, Caribbean and western Atlantic.
Samples were collected at altitudes of eight to 15 kilometres (five to nine miles) before and after two major tropical hurricanes in 2010.
Scientists are still unsure whether the bacteria and fungi they found routinely inhabit the sky, living off carbon compounds, or are continually borne aloft by winds and air currents.
Lead researcher Dr Kostas Konstantinidis, from the Georgia Institute of Technology in the US, said: "We did not expect to find so many micro-organisms in the troposphere, which is considered a very difficult environment for life. There seems to be quite a diversity of species, but not all bacteria make it into the upper atmosphere."
DNA analysis revealed that bacteria made up 20% of particles previously thought to consist of nothing but sea salt or dust.
The findings appear in the journal Proceedings of the National Academy of Sciences.