Bush noted that investigators erred in the early days of the anthrax threat, believing that spores of the potentially deadly bacteria could not escape a sealed envelope.

CNN: Bush reassures Americans about anthrax

Many species of bacteria have been collected from clouds, along with the spores of a variety of fungi.

ECONOMIST: Aerobiology

For these organisms, getting down from a cloud may be more of an ordeal than getting up: spore-releasing fungi tend to shed spores at the times of day and year when thermals are most likely, and bacteria are so small and light that the slightest puff of wind can send them flying.

ECONOMIST: Aerobiology