“Invisible Little Worms” Athanasius Kircher’s Study of the Plague

Living through the devastating Italian plague of 1656, the great polymath Athanasius Kircher turned his ever-enquiring mind to the then mysterious disease, becoming possibly the first to view infected blood through a microscope. While his subsequent theories of spontaneous generation and “universal sperm” were easily debunked, Kircher’s investigation can be seen as an important early step to understanding contagion, and perhaps even the very first articulation of germ theory. John Glassie explores.

April 22, 2020

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Largo Mercatello a Napoli durante la peste del 1656 (The Piazza Mercatello in Naples during the plague of 1656), by Domenico Gargiulo, 1656 — Source.


The plague arrived in Naples in the spring of 1656 — along with a transport of soldiers from Sardinia. At the worst point that summer, thousands were dying there each day. Desperate and frightened citizens flocked in huge numbers to the churches to pray. According to one subsequent account, people “of the highest quality”, as well as the “disheveled”, and presumably the infected, all joined these “confused processions” with the horrific result that “the streets and the stairs of the churches were filled with the dead”.1 By the time it was over in August, as many as one hundred and fifty thousand of the city’s inhabitants had died.

Rome had responded to the news from Naples by policing the seaports and inspecting people and animals at the gates of the city. But by June the disease nevertheless sneaked inside, in this case via a fisherman off the boats at Nettuno. The fisherman was staying at a rooming house in the slums of Trastevere, just across the Tiber from Rome proper, when he began to feel ill. He died just days later, with what were called “evil signs”.2 Possibly these were the infected black buboes or bubones, from which the term bubonic plague is derived, swelling out as big as eggs or apples from under the armpits and from the groin. There may have been boils and abscesses, black or red carbuncles over the entire body, infected and full of pus, or toward the end, a blackening of the skin from hemorrhage. Perhaps he’d lost his senses completely, been vomiting violently, or coughing up bloody sputum.

By order of Rome’s Congregation of Health, soldiers built a wooden barrier around the Trastevere district overnight, shutting in everyone who lived on its narrow streets. Great chains were dropped across the waterways to keep ships from sailing up the Tiber. The city gates were closed. But it was only a short time before more cases were reported, not only in Trastevere but in the Jewish ghetto and other parts of Rome.

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Map of Rome by Matthus Merian, 1642 — Source.

map of rome 1652Scroll through the whole page to download all images before printing.

Vignettes of Rome during the plague of 1656 by Michelangelo Marinari and Giorgio Lambuzzi, ca. 1657. Included are views of buildings used as plague hospitals and places of quarantine (lazarettos), fumigation activities, as well as funerals and processions resulting from the plague. — Source.

The plague was said to exist as a fetid miasma, or corruption of the air from putrid vapors. People thought that epidemics could be occasioned in the first place by celestial activity, a conjunction of malignant Mars with hot and humid Jupiter, for example. Other forms of corrupt air — due to decaying corpses, food, excrement, excessive humidity, stagnant water, emissions from volcanoes or other openings in the earth — could supposedly somehow combine or join with this miasma and make things worse. The poison was believed to stick to fabrics and hair and to penetrate the body through pores in the skin.3

To counteract it, people scrubbed floors and walls with vinegar; burned rosemary, cypress, and juniper; and rubbed oils and essences on their skin. The wealthy left for the country if they could. Vagrants were sent to prison or conscripted to help the sick and scrub the streets of filth. When members of middle-class households were found to be sick, their houses were quarantined, with families boarded up inside. The vast majority of the ill were taken where their exhalations could do the least harm, to quarantined pesthouses, also called lazarettos, after the biblical story of Lazarus — though if you went in, the chances of dying, and staying dead, were high. “Here you are overwhelmed by intolerable smells”, wrote a visitor to a Bologna lazaretto some years earlier. “Here you cannot walk but among corpses. Here you feel naught but the constant horror of death.”4

In late June, schools, courts, markets, and businesses were closed. A general lockdown of the city was ordered for forty days. Doctors walked the quiet streets in seventeenth-century versions of hazmat suits; their beaklike masks were stuffed with herbs and spices. People found violating health regulations or quarantines were jailed, or in many instances put to death. One thirteen-year-old girl, who had run out into the street after a chicken, was hanged, apparently as a lesson to others.

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German engraving of Doctor Schnabel (i.e. Dr Beak), a plague doctor in Rome in 1656, published by Paul Fürst, ca. 1656 — Source.

Although the epidemic continued for more than a year, many of these tactics did help prevent the spread of the disease. The effects of the plague in Rome were much less devastating than in Naples — only about fifteen thousand people died. But living through it was frightening. One figure who did: the fairly eccentric, extremely prolific Jesuit scholar Athanasius Kircher.

From his established place at the Collegio Romano, the flagship institution of his order in Rome, this energetic polymath had thrown himself into the study of just about everything, authoring magnificent seat-cushion-size books on subjects ranging from magnetism to music. During normal times, Kircher often toured distinguished visitors through his museum at the Collegio, where he not only displayed curiosities from around the world (amassed with the help of Jesuit missionaries), but also demonstrated his own magic lanterns, speaking statues, and, as legend has it, a single “cat piano”.

During the summer and fall of 1656, as Kircher remembered it, the “altogether horrid and unrelenting carnage” of Naples was on everyone’s mind, and “each man, out of dread for the ever-looming image of death, was anxiously and solicitously seeking an antidote that would ensure recovery from so fierce an evil.” A melancholic, literary man, he was supposed to be particularly susceptible to the effects of the plague. And yet the prospect of death can sometimes translate into increased ambition, if not a greater desire for immortality. This was the kind of person who pursued his interest in geological matters by lowering himself down into the smoking crater of Mount Vesuvius. So maybe it wasn’t surprising that Kircher would decide to take on the greatest public health challenge of the early modern world.

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Kircher welcoming two guests to the Collegio Romano, a detail from the frontispiece to his Romani Collegii Societatis Jesu Musaeum celeberrimum (1678) — Source.

“In this state of affairs,” he recalled, “amidst the horrible silence of the sad city and in the deepest recess of solitude (for the entrance of the Roman College had been closed), I attempted with sluggish though necessary toil to develop the ideas that I had previously begun to conceive concerning the origin of the plague.”5

During the course of this study, Kircher would become one of the very first people in history to use the microscope to study disease. Perhaps the very first. And he would apply his findings to an argument that was either ancient or brand new, or both, depending on how one looked at it, or the century from which one looked.


As his encyclopedic tome Ars Magna Lucis et Umbrae (The Great Art of Light and Shadow), published in 1645, makes clear, Kircher had been experimenting with lenses and optics for many years prior to Rome’s epidemic. At almost a thousand pages, the volume was meant to provide readers with all they could possibly want to know about light, color, vision, and related matters, and along with instructions for making sundials and projecting images with mirrors, it included a description of what he called a smicroscopus. (Among earlier mentions of the microscope in print, Galileo had written about the use of a telescope adjusted to see things close up. He’d made a gift of one to the experiment-minded members of the Academy of the Lynx-eyed in Rome, who in turn had published descriptions and images of magnified bees. The Linceans, as the members were called, may have been the source of Kircher’s device.)

Kircher’s microscope at that time was perhaps not much more than a short tube with a magnifying lens, or a combination of lenses, fitted inside. But he claimed in The Great Art of Light and Shadow to have seen “mites that suggested hairy bears”6 and minute organisms in cheese, vinegar, and milk. If the living forms that can be seen through a microscope are “so tiny that they are beyond the reach of the senses,” he wondered in later years, “how tiny can their little hearts be? How tiny must their little livers be, or their little stomachs, their cartilage and little nerves, their means of locomotion?”7

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Illustration of Kircher’s microscope, from Musaeum Kircherianum (1709) by Filippo Buonanni, curator of the Kircher Museum in the Collegio Romano — Source.

The polymath gave readers an update of sorts on his work with the microscope in 1658, in his subsequent book about the plague — Scrutinium Physico-Medicum Contagiosae Luis, Quae Pestis Dicitur. (The English translation, A Physico-Medical Examination of the Contagious Pestilence Called the Plague, is often shortened to Examination of the Plague.)

It is “generally known that worms grow from foul corpses”, Kircher wrote in that volume. “But since the use of that remarkable discovery, the smicroscopus, or the so-called magnifying glass, it has been shown that everything putrid is filled with countless masses of small worms, which could not be seen with the naked eye and without lenses.”8

It’s important to say here that for many centuries, spontaneous generation was everyday doctrine. Most people simply assumed that small creatures such as flies, ants, and even frogs and snakes, grew from nonliving matter, preferably swampy or putrescent or excremental. Who could deny, for example, that maggots appeared on rotting meat? And plenty of the old authorities had agreed. Pliny held that insects originated from rotten milk and flesh, as well as from fruit, dew, and rain. Ovid, Plutarch, Virgil, and others believed that bees were born from the dung of bulls. Aristotle believed in spontaneous generation from living matter as well, contending that cabbages engendered caterpillars.9

Kircher explained that he’d performed an entire series of “experiments” related to this apparent phenomenon which he invited readers (with access to a similar instrument) to try. For example:

Take a piece of meat, and at night leave it exposed to the lunar moisture until the following day. Then examine it carefully with a microscope and you will find that all the prutridity drawn from the moon has been transformed into numberless little worms of different sizes, which in the absence of the microscope you will be unable to detect . . .

The same basic thing happens, he said, if you take a bowl of water sprinkled with dirt from the ground and expose it to the sun for a few days: “You will see . . . certain vesicles which are quickened into exceedingly minute worms”, which eventually become “a vast number of winged gnats.”

It also happens “if you cut a snake into little pieces, soak them in rain water, expose them for some days to the sun, bury them for a whole day and night in the earth, and then, when they are soft with putridity, examine them with a microscope” — except here what you see is the decaying mess swarming with little “snakes”.10

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Illustration of snakes from Arca Noë (1675), Kircher’s book on the biblical story of Noah’s Ark — Source.

Based on these “incontrovertible experiments”, there was really no doubt in his mind about what he thought he knew: that tiny lower creatures grew from the decay of other living things. And it occurred to him that, at least as he and everyone else believed, the plague also arose from decay and putridity.

It was with this in mind that, on several occasions, Kircher examined the blood of plague sufferers under his microscope. “The putrid blood of those affected by fevers has fully convinced me”, he wrote. “I have found it, an hour or so after letting, so crowded with worms as to well nigh dumbfound me.” His assertion: “Plague is in general a living thing”.

The arguments in Examination of the Plague are hard to follow, but something called panspermia, or universal sperm, was certainly involved: Kircher believed that “seeds of a vegetative and sentient nature” are “scattered everywhere among the elemental bodies”, and that decomposing matter acted as a kind of fertilizer or necessary ingredient for the generation of new forms. In the case of the plague, he suggested that when the tiny semina, or seeds, or “corpuscles”, that emanate from all natural things become corrupted by putrescence, they become the minute carriers of the disease. “Corpuscles of this kind are commonly nonliving”, he explained, “but through the agency of ambient heat already tainted with a similar pollution, they are transformed into a brood of countless invisible little worms.”

According to Kircher, these “propagators of the plague” are “so small, so light, so subtle, that they elude any grasp of perception and can only be seen under the most powerful microscope.” Therefore, they “are easily forced out through all the passages and pores” of plague victims’ bodies and of the sick and “are moved by even the faintest breath of air, just like so many dust particles in the sun.” They are then “drawn through the breath and through the sweat pores of the body, from which later such fearful symptoms and effects result.”11

Over the next several years, Kircher took his theorizing to new levels; in his view, universal sperm was essentially the life force. But his beliefs, at least about spontaneous generation, were rather humiliatingly debunked in 1668 by a physician in the Medici court named Francesco Redi. In his Esperienze Intorno alla Generazione degl’Insetti (Experiments on the Generation of Insects), Redi describes one of the very first controlled experiments ever documented: “I put a snake, some fish, some eels of Arno, and a slice of milk-fed veal in four large, wide-mouthed flasks; having well closed and sealed them, I then filled the same number of flasks in the same way, only leaving these open.”12

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Illustrating showing how a fly is born from a maggot found in a cherry, from Francesco Redi’s Esperienze Intorno alla Generazione degl’Insetti (1668) — Source.

Maggots soon began to appear on the flesh in the open containers, but not in the closed ones. He experimented with a number of variations, sometimes covering the flasks with “a fine Naples veil”, but no maggots or worms or anything at all ever appeared inside the covered containers. He also dutifully followed Kircher’s instructions for breeding bees in the dung of an ox, for breeding scorpions in dead scorpions, and for breeding flies in dead flies. He never had any luck. With no insects appearing in closed flasks, Redi was convinced that “no animal of any kind is ever bred in dead flesh unless there be a previous egg deposit.”13

That was really all it took to to disprove an age-old notion about the genesis of lower forms of life. Redi referred to Kircher somewhat patronizingly as “a man of worthy esteem”, but it had never occurred to Kircher to regulate his experiments this way — to put a lid, as it were, on his container. Just a few years later Redi published another set of findings; this time they exposed Kircher’s gullibility about the healing powers of something called the snakestone.


In 1932, C. Clifford Dobell, a leading biologist with a specialty in protozoology, described Examination of the Plague as “a farrago of nonsensical speculation by a man possessed of neither scientific acumen nor medical instinct”.14 But many others have credited Kircher with a major discovery. In the words of another twentieth-century medical authority, Kircher “was undoubtedly the first to state in explicit terms the doctrine of ‘contagium vivum’ as the cause of infectious disease”15 — in other words, that Kircher discovered microorganisms and was the first to propose the germ theory of contagion.

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Views through a microscope of bugs found in the refuse of plants as well as vermiculite, pepper, and salt, featured in Musaeum Kircherianum (1709) by Filippo Buonanni, curator of the Kircher Museum in the Collegio Romano — Source.

If that’s true, however, then his articulation of germ theory was predicated on notions no modern scientist would be caught dead advancing. Besides, the concept of universal seeds went back to the Greek philosopher Anaxagoras, and the idea that disease is living turns out to be both ancient and mystical. (Today, by the way, although bacteria — like the kind that cause the plague — are understood to be living organisms, viruses — like Covid-19 — are not considered to be “alive”.)

A lot of what Kircher wrote about disease came from Lucretius, the disciple of Epicurus from whom Pierre Gassendi got his modern-sounding ideas about atoms. In his epic poem, De Rerum Natura (On the Nature of Things), Lucretius wrote that “there are many seeds of things which support life, and on the other hand there must be many flying about which make for disease and death.” They can come “down through the sky like clouds and mists, or often they gather together and rise from the earth itself, when through dampness it has become putrescent.”16

Among other sources, Kircher also borrowed heavily from, but doesn’t cite, a sixteenth-century writer most famous for a verse treatise called Syphilis, or the French Disease. (Understandably, no one wanted to take the credit for syphilis, which reached epidemic proportions during his time — to Muslims, it was the disease of the Christians, to the English it was the French pox, to the French it was the Neapolitan disease, and to the Italians it was of Spanish origin.) The author, a physician from Verona named Fracastoro, worked out a theory of contagion involving transmission of “imperceptible particles”, infected and self-propagating, that he called seminaria, or “seedbeds”, sometimes translated as “germs”.17

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Frontispiece to a 1669 Rotterdam edition of Kircher’s Scrutinium Physico-Medicum Contagiosae Luis, Quae Pestis Dicitur, his portrait hovering above a plague-ridden victim (who, with the wolf, might be a representation of Rome itself) — Source.

What did Kircher actually see when he examined the blood of plague patients? He claimed his microscope made “everything appear a thousand times larger than it really is”, but he didn’t mean that literally. It’s not possible that he saw plague bacilli, which are one six-hundredth of a millimeter long. Even if he employed a form of compound microscope (which consists of a series of lenses), any organic specimen might have looked like a mass of tiny worms.

And yet most of Kircher’s readers had never looked through a microscope at all. Only one or two treatises on the subject had ever been published, and Examination of the Plague caused a kind of sensation within the network of scholars and philosophers known as the Republic of Letters. A doctor in Dresden compared Kircher’s brilliance to the shining of the sun. An anatomy professor in Jena informed Kircher that “the reputation of things Kircherian” had “spread through all of Europe”. In thanks for his copy of the book, a missionary in New Spain sent Kircher chocolate and peppers “that have the name of Chile”.18

Eventually, whether or not Examination of the Plague hit upon one of the core tenets of epidemiology, it did inspire a great deal of experimentation with the microscope, and it influenced thinking that in turn led to (actual?) discoveries about the way disease is spread. You might therefore say that the epidemic of 1656, itself, led to such discoveries, although the gains came at a monstrously heavy price.

One final note: Kircher’s eager readers may have been under the incorrect assumption that he had something to do with stemming the plague in Rome. And on the question of prevention and cure he did provide his considered opinion, but this one can’t be mistaken for an early expression of modern medical thought: he believed that, short of leaving the area, an amulet made of the flesh of a toad, or of dried toad powder, and worn over the heart, was probably the best antidote.19

John Glassie has written for The Believer, The New Republic, The New York Times Magazine, The Paris Review Daily, and The Washington Post, among other publications. This essay was adapted from his biography of Athanasius Kircher, A Man of Misconceptions: The Life of an Eccentric in an Age of Change, which was a New York Times Book Review "Editors' Choice".

The text of this essay is published under a CC BY-SA license, see here for details.