The Societal and Intellectual Barriers to a Unified Microbial Theory in Ancient Greece and Rome

Written by Miranda Ginder


1§1 Microbiology, as it is today, seems to be a modern creation beginning with Antonie van Leeuwenhoek. He peered into his self-built microscope at pond water and observed microbial life, which he called ‘animalcules’[1] However, since the time of the Presocratics, the invisible world of microorganisms has been imagined theoretically and discussed. Several different philosophical and medical schools proposed invisible, seed-like agents of disease that can spread through the air, water, and direct physical contact. The ancient Greeks gained information about the unseen from the seen, observing plagues and sickness to understand the complex world of microbes surrounding them. Despite all of the evidence acquired, the fragments of microbial truth remained that: fragmented. The medical and philosophical schools never assembled the available evidence to draw conclusions about microbial life.

1§2 Earlier scholarship on Greek microbial theory has clarified several important aspects of this area of inquiry. For example, the analogy of bacteria to seeds is particularly significant, as Vivian Nutton clarified in his article, “The Seeds of Disease.”[2] He explained that the choice of the word “seed” implies specific characteristics the ancient author intended to impress upon his audience. A seed shows the disease-causing agent is alive, very small, and has the ability to grow and eventually reproduce. These basic aspects prime the reader to think of disease causing agents in this way. Moreover, he collected most of the ancient evidence for Greek microbial theory, and his collection of evidence has been instrumental for the analysis presented here.

1§3 In this paper, I address the inability of the philosophical and medical schools to synthesize a robust theory of microbial life. However, in order to account for this deficiency, I will present and analyze evidence which demonstrates that the obstacles to a full understanding of microbial existence and influence. The schools of philosophy and medicine relevant to this argument include: the Atomists, Aristotelians, and Hippocratics, and the medical schools include the Methodists, Rationalists, and Empiricists. The writings from each philosophical or medical school contain fragments of a microbial theory. Similar terminology is found in the writings from each school as well, all of which employ such terms as ‘miasma’, “putrification of air,” and “seeds” in essentially the same manner, in that they describe microbial life and their physiology. This volume of evidence could have been collected by the philosophical and medical schools, but the schools were not able to synthesize a complete theory of microbial life, that is, a theory which brought together the all of the available evidence of microbes. For this paper, I have defined completeness as a coherent, consistent concept based on all the available evidence, with a desire to further investigate the theory.

Presentation of the Evidence

2§1 Early in Greek history, Hippocrates and his followers practiced medical philosophy and treatment from 460 to 370 BCE. They collectively created the Hippocratic corpus, and their work is considered to be the true beginning of scientific medical theory. Their most important theory explained how the imbalance of the four humors (black bile, yellow bile, phlegm, and blood) led to a disease state within a patient. The Hippocratics included in their theories concepts relating to microbial infection, but they did not go any further than this. They detailed how the seeds of disease could cause a breakdown or change in a person’s humors and how this caused the overall illness. The infection and growth of microbes deteriorated or altered the distribution and composition of the four humors which led to the disease state. In Nutton’s article he states that for the Hippocratic authors, “the seeds of disease act only as an initial cause: they are not the disease, any more than a blow to the head or a poisonous mushroom. They merely trigger a situation which eventually may lead to a humoral disorder and a bodily malfunction….”[3] This idea links the concept of microbial life to changes within the humors, and these factors then create a disease state. Helen King further discusses the concept of humoral changes in her book Greek and Roman Medicine. She explains that the body was “in balance” and healthy when the constituent humors were in equal proportion. Disease states were the result of a body out of balance, and to return a body to balance, the treatment had to be opposite the symptom.[4] For example, if a man was cold, providing him extra heat would return humoral balance. This cause and effect relationship mimics the physiological response now understood and studied by medical professionals today. Of specific interest is how the amount of white blood cells and lymphocytes increases during active infection, much like the humoral changes theorized by the Hippocratics. Within the Hippocratic text On the Nature of Man there is a section with content about how the “bad air” contains unhealthy exhalation or excretion from a sick individual into the environment. The author writes, “But when an epidemic of one disease is prevalent, it is plain that the cause is not regimen but what we breathe, and that this is charged with some unhealthy exhalation.”[5]

2§2 The concept of contagiousness, or the infectious ability of microbes, originated from Aristotle in the work Problems, VII.8: 887A.[6] Within the text there is an investigation into the difference in disease spread, particularly why some conditions are transmitted from individuals and others are not. The text states:

“Why is it that those who come into contact with phthisis or opthalmia or scurvy become affected by them, but there is no contagion from dropsy or fevers or apoplexy and the rest? … In phthisis is the contagion due to the fact that phthisis makes the breath weak and laboured, and those diseases are most quickly contracted which are due to the corruption of the breath, as is seen in plagues? He therefore who comes into contact with the sufferer inhales this corrupted breath, and so himself becomes ill, because the breath is unhealthy; and he catches the disease from one person only, because that person exhales this particular breath, which is different from that which others exhale; and he catches the same disease, because, in inhaling the breath by which he becomes infected, he is inhaling just such breath as he would if he were already suffering from the disease.”[7]

2§3 The conclusion is that some exudate, or diseased air discharged by the lungs, is released because of a specific illness, and that other illnesses do not release exudate. The pestilence or putrification is a characteristic of each disease. To Aristotle, this explained why fevers can infect individuals, but epilepsy cannot. Additionally, it establishes a justification for quarantine, because if someone becomes ill with a recognizable illness, and that illness releases putrification, then the person must be isolated from the healthy population.

2§4 The Hippocratics and Aristotle do not represent all the testimony to microbial theory in the ancient world, as Lucretius also presented an Epicurean theory on the subject. Having composed his poem On the Nature of Things around 55 BCE, Lucretius was an Epicurean, a philosophy which assumes for its physical theory the existence of small, indivisible particles called atoms. These atoms move randomly, cause events to occur, and are the basic unit of matter. In his works, he discussed the function of microbial life and had unique ideas on the structure of microbes. As an Atomist, he hypothesized the means of action and capabilities of microbes were decided by the atomic shape and combination of those atoms. A physician contemporary of Lucretius, Asclepiades, seems to have applied atomic thought to medicine, and thereby dismissing humoral theory. He may then have been a bridge between Epicurean philosophical ideas about atoms and the medical school known as Methodism.[8] In Lucretius’ work On the Nature of Things at lines 770–776 of Book Six, Lucretius writes: “in earth are things… many with the power to infect us with disease and hasten death. And that some are better for some… because of their different substances and different textures, one from another, and the outward shapes”[9]. By relating the “outward shape” to the infectious capability of an atom, he is alone is discussing the structure of something like microbes, but without the aid of advanced optics, he could go no further. Lucretius also stated that it is the seeds in the air that cause it to become putrid, which associates putrification and the seeds of disease. He states in lines 1093–1097: “atoms…cause of disease and death…When they chance to rise and trouble the sky, the air becomes diseased”[10]. He essentially expanded the means of transmission by stating that contaminated food, crops, and water were able to become infectious if seeds were present in them, which he stated in On the Nature of Things, 1124–1130. Lucretius also isolated a location favorable for microbial growth, that being soil, which can house a phenomenal diversity and load.

2§5 Other evidence for the known existence of microbial life among the ancient Greeks comes from the Roman authors Varro and Columella. Varro lived between 116 and 27 BCE and is a predecessor to Columella, who lived between 4 BCE and 70 AD. Each man did not provide a microbial theory based in a philosophical system; nonetheless, they provided valuable insights into microbial functions. Though each man was a Roman, each depends upon Greek sources: Varro, for example, was well-versed in Anaxagoras, and knew about the “invisible” things theorized by him.[11] Both Varro and Columella discussed microbial life through the analysis of agriculture and swampland. Varro’s work On Agriculture (XII) discussed microbial life using the term ‘animalcula’[12]. Much like the seed analogy, it demonstrates their basic assumptions about bacteria, in that they are small and, conceived as living things, and able to grow and reproduce. He used the ‘animacula’ theory as a reason to not live on or near a swamp; the ‘animacula’ lived in those environmental conditions, and they were best avoided. Similarly to Varro, Columella discussed the avoidance of swamps, but, unlike Varro, he also brought into existence the concept of what we would call a vector. A vector is an animal or human host in which a microbe can reside, reproduce, and spread.[13] Columella first associated sickness with mosquitos, and mosquitos with swampy land. He explains in On Agriculture that “neither should there be any marsh-land near the buildings, and no military highway adjoining; for the former… breeds insects armed with annoying stings, which attack us in dense swarms; then too it sends forth plagues of swimming and crawling things… infected with poison by the mud and decaying filth, from which are often contracted mysterious diseases whose causes are even beyond the understanding of physicians.”[14] These associations align with modern knowledge, as mosquitos are a host for plasmodium, the infectious agent of malaria.[15]

2§6 An important source for ancient microbial theory is Galen’s corpus of medical writings; he outlines the functional aspects of disease causing seeds, and he explains their role in human health. Galen himself was the private physician of Marcus Aurelius and forged new perspectives and discoveries in medicine from 129 to around 210 AD. Among Galen’s texts, the discussion of seeds of disease occurs in three different texts: On Initial Causes, On the Different Fevers, and Epidemics I. All of these sources are helpfully collected in Nutton’s article “The Seeds of Disease”.[16] In On Initial Causes, Galen modifies the Aristotelian account of causes of disease, explaining that the initial causes combine with the antecedent causes to create the cohesive cause.[17] The initial cause is a physical predilection or susceptibility to a disease state, and the antecedent cause is the environmental shift that, in conjunction with initial causes, creates a disease state. These two causes discussed dually are called cohesive causes by Galen. It demonstrates that a predisposition and a complicating factor, like disease causing seeds, can then unify and create a disease state in a body. The morbidity of seeds and their infectious route are explained in Galen’s On the Different Fevers.[18] He details how some people, due to their previous state of health, may be more susceptible to contracting a fever from seeds of disease. Additionally, the breath of the sick can contain these seeds, and their “putrid exhalations” are able to spread the infection from the sick to the healthy. Lastly, in his Epidemics I, seeds are shown to be a direct cause of disease, and if treatment is ended early, the seeds may not be entirely removed from the body.[19] He theorizes that “corrupt humours” remain within the body unless a proper regimen is maintained. This is able to cause recurrent infections, a concept that describes an obstacle remaining today in modern outpatient treatment.

2§7 These sources establish the information known about microbial existence in Greco-Roman antiquity. According to the sources, they are small, are capable of growing and reproducing, often grow in swamps, are contagious, do not account for all disease, and their structure and function is based on atomic shape. They also understood that some individuals are more susceptible, that they interfere with normal physiological conditions, can be transmitted through the air and infected food and water, and can be carried by other animals. With this body of evidence, it may seem curious that these positions did not coalesce into a cohesive microbial theory. But, as I will show in the next section, the disagreement and divergence of opinion among the schools of philosophy and medicine barred a unified understanding of microbiology.

Theoretical Obstacles to a Unified Microbial Theory

3§1 To my mind, there are three probable explanations for the limited advances in the area of ancient microbial theory. First, the reliance upon discussion as a method of investigation among the schools could only take their discoveries so far. In the conversational exchanges between schools, the discussion style is most often that of debate. A central question is addressed by representatives of each school, with the goal being intellectual dominance achieved by a defender of one or another school. These exchanges often were intense and placed the schools in direct conflict, and from that hostility the differences between the schools were highlighted. Discussions in this manner do not create an atmosphere conducive to the collaborative synthesis of opposing theories.

3§2 An example of this discussion style and the hostility created among its users comes from Table-Talk,[20] a philosophical text attributed to Plutarch. The debate focused on the seeds of disease, and included Philo, Diogenianus, and Plutarch discussing the possibility of the creation of new diseases. Philo defended the concept of new diseases, believing that they come into our world by atoms moving in the spaces between worlds.[21] As evidence, he used diseases that had never been described before. For this view, he was attacked by Plutarch and Diogenianus. Diogenianus explains, “There cannot be a new disease without a cause, for this would introduce into the world, contrary to natural law, a coming-to-be from non-being…”[22] This position directly contradicts and vehemently disagrees with Philo’s stated view. These types of fiery exchanges are not unusual in the surviving works of ancient Greek and Roman scientific thought, and at worst they created a barrier to unifying a microbial theory.

3§3 A second barrier to a complete microbial understanding is the rigidity of philosophical ideas within each school. During the debate-style discussions analyzed previously, it becomes apparent that each school refused to waver from the mandated claims and stance. Each individual member of the school fully adopted the philosophy of the school, in order to determine the most rational approach for the particular school. Essentially once a philosopher adopted one among the philosophical schools, he was expected to adhere to the shared dogmas of the school. This rigidity created an inability to reconcile ideas among the schools of philosophy, and may have contributed to the persistent disorganization in the theoretical conceptions of microbial life. A collective and united thought pattern helped each school have a uniform philosophy, but may have hindered the ability to consider outside ideas. Instead of contemplating and analyzing the theories of other philosophical schools, an individual was trained to combat the other schools’ concepts with the arguments from the individual’s school.

3§4 Few examples of direct debate concerning microbial organisms survive from antiquity, but analysis of texts can demonstrate the dissention and discord between schools. Discussion within a text is often comparative of two or more philosophies, with the author striving to prove that their philosophy is the only viable position, as Galen’s On Sects for Beginners, for example, demonstrates. The purpose of the writing was to inform readers on the criticisms the medical schools had toward each other. In Chapter Five, Galen opens with a continued discussion of the importance of experience compared to rational reasoning as applied to patient treatment. He says,

“But the dogmatists have levelled various criticisms against empiricism. Some have said that this kind of experience is unrealizable, and others, that it is incomplete, while a third group has claimed that it is not technical. The empiricists, on the other hand, have criticized rationalist reasoning as being plausible, but not true. Hence the account which both of them give is twofold and turns out to be rather lengthy, as they raise particular criticisms and defend themselves against them.”[23]

This adherence to and defense of a disposition associated with a medical school does not directly refute the competing views in the dialectical style employed in Plutarch, for example, but would place physicians, like philosophers, in conflict; as it is known that philosophers read the works by other philosophers, so too would physicians read works by physicians from opposing schools. The focus, however, was generally on differences rather than similarities.

3§5 Among the inter-school discussions, the debaters seem to be more concerned with the logic behind the conclusions and the specific, refutable claims of each other’s philosophies rather than the content of the issue at hand. Naturally, this makes sense; the debates often were on philosophical matters. The subject matter was more concerned with the universal assumptions instead of the example that was used as a medium for discussion. Other examples of this tendency include the nature of man, what constitutes a well-lived existence, the origins of humanity, and the soul. This unfortunately meant that even when microbial life was discussed, it was a means for debate and differentiation rather than unification. For example, Plutarch’s Table-Talk delves into this topic, and one specific area highlights this attention to small details rather than the larger topic. Diogenianus rebutted Philo on the idea that new diseases can come to be, because that would break “natural law.”[24] He opposed this idea because Philo followed the Atomist tradition, with which Diogenianus disagreed. Essentially, all of Philo’s innovations were immediately condemned because of the previous universal assumptions of Atomism.[25] This type of attitude and combativeness due to the different schools’ core principles served to divide investigators into questions around microbial existence. There seems to have been an issue with supporting the ideas while disagreeing with the specifics of another philosopher or school. The differences in theoretical principles and the prioritization of theoretical consistency over empirical discovery hindered the progress of knowledge concerning microbes. Though it served the intended purpose, it also seems to have been a lost opportunity, as the available evidence could have been conglomerated into a more consistent microbial theory.

3§6 For understanding the philosophical discussion of microbial life, no text is as important as Plutarch’s Table Talk. Because the text proceeds dramatically, setting different personae adherent to different theoretical positions against one another, there is need for a further inquiry into the motive of the text. Judith Mossman explains in her essay “Dionysus and the Structure of Plutarch’s Table Talk” that the overall purpose of the text is an “expression of Plutarch’s paideutic” and that the point of asking questions is to receive a variety of answers.[26] This is why there is such a rich diversity of topics and positions taken toward them throughout the text. The discussion of new diseases, then, can be understood as one question which receives diverse and plentiful answers. Eleni Kechagia goes further, explaining in her essay “Philosophy in Plutarch’s Table Talk” that the text was to be used for philosophical learning.[27] Though the text was originally addressed to Sossius Senecio, Plutarch desired for the piece to be “food for the soul” through intellectual conversation.[28] It would teach beginners how to “do” philosophy through tools and examples.[29] He showed the method for approaching philosophical problems by discussion and reasoning skills.[30] In the end, he wanted readers to find plausible explanations[31] and to learn how to use tools to understand nature and reason and explain natural phenomenon.[32] Essentially, it provided tools and training to the untrained mind, and microbial life proved an example that fit within the criteria for the text. It provided both diversity of discussion and a framework to provide tools and real-life examples to beginner philosophers. Therefore, Plutarch uses microbes only as an example and does not show how the divergent theoretical positions could be harmonized into a more complete and comprehensive theory.  It is for these reasons that the discussion of microbial life is included within the text, Table Talk.


4§1 Within this paper, evidence of microbial existence gathered by the ancient Greeks and Romans and their lack of collaborative ability has been demonstrated. But today we are given to believe that collaborative effort towards scientific understanding benefits both the participating individuals and the conclusions gleaned from inquiry. For example, the Office of Research Integrity, a division of the US Department of Health and Human Services, directs and oversees all research activities funded by the US Public Health Service. They discuss the benefits of collaboration in their article, “Need for Collaboration”, and state that collaboration has “stoked the pace of research and encouraged the development of innovative and groundbreaking strategies in investigating increasingly novel, complex and convoluted areas.”[33] Discussion between related disciplines can prompt creative solutions to pre-existing problems, and by utilizing new perspectives, more insight into biological understanding can be gained. Multidisciplinary are thought to have approaches have far-reaching consequences, including “…social and economic benefit to society, science, and private industry.”[34] Although these conclusions may be more relevant to modern times, the advantageous nature of collaboration cannot be ignored. The addition of collaborative initiative among the ancient philosophical and medical schools could have not only revolutionized microbial comprehension, but increased knowledge in innumerable fields.

4§2 In some respects, these strict divisions and partisan positions among the philosophical and medical schools served their purpose: they clearly identified the philosophical school and aided in refining the core principles of the philosophy. The Greeks may have never intended for their microbial discussions to be anything more than exercises of the mind, and were so utilized. Therefore, the function of microbial theory for the medical and philosophical schools served the intended purpose, but underlying that purpose was a greater opportunity for the intellectual and social unity of science. This is not to imply this is a fault or oversight in Greek philosophy and medicine. On the contrary, discussions and rebuttals between the schools led to innovative thoughts and advances in microbial theory. Theories of the microscopic were not limited to one school, and through the diverse methods of analysis, many schools contributed their own piece to the microbial puzzle. Without the differences between schools, it may not have been possible to have a rich microbial landscape. The evidence and concepts originally synthesized by the medical schools and philosophers created a framework for later scientists, like Leeuwenhoek, to take these insights as a foundation for further inquiry.


[1] Dobell and Leeuwenhoek 1960:37-41

[2] Nutton 1983:1-3.  This article, appearing in MedHist, features an in-depth investigation into microbial knowledge from Anaxagoras through the Medieval Ages. I heavily drew on this source.

[3] Nutton 1983:15.

[4] King 2009:12

[5] Jones 1931:9-10; Hippocrates On the Nature of Man 9

[6] Aristotle Problems VII.8: 887A. Alexander of Aphrodisias was a commentator on Aristotle around 200 AD, and his words are often included within Aristotle’s text. This most likely occurred due to errors during the transcription of new copies of the text. Within this section, Alexander has not made any evident editing or commentary, and I have therefore treated it as a text solely attributed to Aristotle.

[7] Aristotle Problems, VII.8: 887A

[8] Santacroce et al. 2017:1-7

[9] Copley 1977; Lucretius On the Nature of Things VI.

[10] Copley 1977; Lucretius On the Nature of Things VI. 1093­–1097

[11] Nutton 1983:11 Anaxagoras, an important philosopher of the fifth century BC, had theorized about a world too small to see in one of his fragments: “Appearances are a vision of the invisible” (63 [F23]; Graham 2010). This concept of unobservable but existential objects may have influenced other, later authors.

[12] Varro On Alexandria XII

[13] Center for Disease Control and Prevention 2018

[14] Columella On Agriculture I, Preface, 6-7

[15] Center for Disease Control and Prevention 2018

[16] Nutton 1983:1-21

[17] Galen On Initial Causes 100-108

[18] Galen On the Different Fevers I.3

[19] Galen Epidemics I III.7

[20] Plutarch Table-Talk VIII.9

[21] Plutarch Table-Talk VIII.9.1

[22] Plutarch Table-Talk VIII.9.2

[23] Galen On Sects for Beginners

[24] Plutarch Table-Talk VIII.9.2

[25] Plutarch Table-Talk VIII.9.2

[26] Mossman 2017: 102-112

[27] Kechagia 2011:83

[28] Kechagia 2011:85

[29] Kechagia 2011:93

[30] Kechagia 2011:95

[31] Kechagia 2011:98

[32] Kechagia 2011:99

[33] Office of Research Integrity 2018

[34] Office of Research Integrity 2018


“About Malaria Biology.” Centers for Disease Control and Prevention.

Ash, H.B., ed. and trans. 1941. Columella: On Agriculture. Loeb Classical Library. Cambridge, MA.

Barnes, Jonathan, trans. The complete works of Aristotle : the revised Oxford translation.  Princeton, NJ: Princeton University Press, 1984.

Copley, Frank Olin, trans. 1977. Lucretius. On the Nature of Things. New York.

Dobell, Clifford, and Antony Van Leeuwenhoek. 1960. Antony van Leeuwenhoek and His Little Animals. New York.

Graham, D. W. 2010. The Texts of the Early Greek Philosophers: The Complete Fragments and Selected Testimonies of the Major Presocratics. Cambridge.

Hooper, W.D., and Ash, H.B., eds. and trans. 1934. Cato and Varro: On Agriculture. Loeb Classical Library 283. Cambridge.

  1. H. S. Jones, W.H.S. ed. and trans. 1939. Hippocrates, with an English translation by W.H.S. Jones. London.

Kechagia, Eleni. 2011. Philosophy in Plutarch’s Table Talk. Oxford.

King, Helen. 2009. Greek and Roman Medicine. London.

Minar, Jr. Edwin L, ed. and trans. 1961. Plutarch: Moralia, Vol IX. Loeb Classical Library 425. Cambridge, MA..

Mossman, Judith. 2017. Dionysus and the Structure of Plutarch’s Table Talk. Leuven.

“Need for Collaboration.” Responsible Conduct in Collaborative Research, US Department of Health and Human Services.

Nutton, Vivian, “The Seeds of Disease: An Explanation of Contagion and Infection from the Greeks to the Renaissance.” Medical History, 27 (1983): 1-34.

Santacroce, Luige, Bottalico, Lucrezia, and Charitos, Ioannis. Greek Medicine Practice at Ancient Rome: The        Physician Molecularist Asclepiades. Medicines, 2017, 4, 92.

Walzer, Richard, trans. 2000. Galen. Three Treatises on the Nature of Science. Indianapolis.



Leave a Reply