germs can fly--4/1/2026

01 Apr 2026

germs can fly--4/1/2026

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Today's selection-- from Air-Borne by Carl Zimmer. An atmospheric kingdom of life:

“Yellow fever returned to Philadelphia a number of times over the next decade, and each time Rush became more convinced that 'the miasmata from our atmosphere' were to blame. Writing in 1805 to former president John Adams, Rush declared the outbreaks were bringing him victory over his contagionist rivals. 'A new era has begun in the science of medicine in our city since the appearance of the yellow fever among us,' Rush wrote.

“These were not the declarations of a crank. When Rush died in 1813, he was hailed as the greatest American physician. 'Like a primal fixed star, amid the host of heaven, he shone with a lustre wholly his own,' a minister declared at a memorial for Rush. 'He has done more good in this world than Franklin or Washington,' John Adams wrote.

“After his death, Rush inspired a new generation of anticontagionists. They held control of the medical establishment in both the United States and Europe, and they roundly condemned contagionism as bad for both health and business. Quarantines needlessly slowed down trade by forcing ships to stay isolated for weeks before unloading their goods. In 1824, the British physician Charles Maclean declared that quarantines were worse than useless: by trapping people in the very places where the air carried death, they were "little short of willful murder.

“In the early 1800s, contagionists remained profoundly ignorant about the microorganisms they believed spread diseases. They had yet to link any particular germ to any particular disease. Someone like Maclean could continue to mock the very idea that a germ could spread a disease. In an age dedicated to science, nothing seemed less scientific. 'This unknown and incomprehensible power is endowed with the faculties of self-generation, self-annihilation, self-resuscitation, self-transportation, self-propagation, and an immense variety of other capabilities, no less wonderful, which it condescends to exercise for the destruction of mankind,' Maclean sneered.

Scanning electron microscope image of Vibrio cholerae, the bacterium that causes cholera

“Even as Maclean attacked the contagionists, the world of microorganisms became less unknown, less incomprehensible. In 1824, when Maclean launched his withering attacks, the German naturalist Christian Gottfried Ehrenberg was in Africa on an epic series of journeys that would take him across Arabian deserts and Siberian forests. Along the way, he made the first systematic survey of microorganisms.

“Back in Germany, Ehrenberg organized his tiny creatures into a new classification. He named one group of species Bacteria. He recognized that microorganisms were the most abundant form of life on Earth, thriving everywhere he looked. He found fossils of microorganisms in chalk and limestone–or, to be more precise, he realized that chalk and limestone were made of their long-dead cells. 'Of small things worlds are built,' Ehrenberg once declared.

“While Maclean sneered at the idea of self-transporting germs, Ehrenberg found evidence that they could take spectacular journeys. They could even fly. Ehrenberg's first clue came from strange black sheets called paper meteorites, which were believed to have fallen from the heavens. They were so strange that museums kept them in their collections along with other oddities of nature. When Ehrenberg examined the paper meteorites, he recognized what others before him had missed: the sheets were actually dried mats of microorganisms. He speculated that they had started out growing in intertidal pools on beaches. Storm winds came along, peeling them from the coast and lofting them high in the atmosphere, where they drifted for hundreds or thousands of miles before dropping down to Earth.

"Thinking back on the microorganisms he had encountered in so many places, Ehrenberg reasoned that storms might also sweep up microorganisms from deserts and ponds. If you knew what to look for, you could even find evidence for flying microorganisms in historical records. There were many accounts of blood raining down from the sky. In The Iliad, Zeus sends a red downpour as a sign of coming slaughter. In the fourteenth century, a shower of blood in Germany presaged the arrival of the Black Death. Ehrenberg believed that that blood rain was real, but not really blood. It was instead a red mix of dust and microorganisms raised into the sky and then washed down in storms. 'How many thousand millions of tons of microscopic life forms might have been lifted and have fallen like meteorites onto the Earth since Homer's blood rain!' Ehrenberg exclaimed.

“In 1844, Ehrenberg enlisted Charles Darwin in his search for flying life. Ehrenberg knew that the young British geologist had visited some of the most remote places on Earth during his voyage on the Beagle. He wondered if Darwin might have had any samples to spare from places like the Galapagos Islands. Darwin was happy to help, and he added a gift he thought might interest Ehrenberg: a vial of mysterious red dust.

“Darwin had collected it twelve years earlier, when he was just starting on his five-year voyage on the Beagle. As the ship sailed across the Atlantic toward Brazil, he noticed one day that the sky was turning hazy. The cause, Darwin later wrote, was 'the falling of impalpably fine dust.' It dirtied the ship as it settled on every surface, and the crew of the Beagle complained as the dust stung their eyes. 'Vessels even have run on shore owing to the obscurity of the atmosphere,'Darwin wrote.

“Darwin swept a smidgen of dust into a packet and stored it away in his quarters on the ship for later study. When he returned to England in 1836, he brought the dust back home along with his vast collection of fossils, rocks, and preserved animals and plants to his country estate. As Darwin developed his theory of evolution, he did not have time to even peek at the packet. Ehrenberg's request tickled his memory. He tracked down the dust and sent it to Berlin.

“Darwin had suspected that the dust had come from some distant volcanic eruption, but Ehrenberg saw no evidence for that under his microscope. Instead, he was looking at life. Most of the dust was composed of microorganisms covered in shells of silica. Some of them reminded Ehrenberg of freshwater species that lived in South America.

“In June 1844, Ehrenberg sent Darwin his preliminary findings. 'I am truly astonished at this,' Darwin wrote back.

"'I will feast on them for a long time yet,' Ehrenberg promised him. Darwin persuaded other naturalists to send their supplies of Atlantic dust to Berlin. All told, Ehrenberg discovered sixty-seven species of microorganisms in the samples. He puzzled with Darwin over where they had traveled from. Ehrenberg favored South America as the source, while Darwin favored Africa. In 1846, Darwin published an account of some of the results of Ehrenberg's analysis.

"Fundamentally, it didn't matter if Darwin or Ehrenberg was right. Either way, the microorganisms must have soared around a far swath of the planet, perhaps flying above the clouds. They were one more piece of evidence that Ehrenberg could marshal to show how far microorganisms could travel. In the words of one Victorian scientist, Ehrenberg's studies 'proved the actual existence of an atmospheric kingdom of life.'"