Germs and Disease

Today even children know about germs and disease. But, believe it or not, it took centuries to recognize the connection. So what is a germ and why did it take centuries to connect germs and disease? To start with, germs are invisible, and before the microscope, they were unknown.

There are four types of germs: bacteria, viruses, fungi , and protozoa. Relatively few in each of these categories cause disease. In general, complex life forms depend on them. But back to disease.

It’s important to know the differences between these categories. For example, colds and flu are caused by viruses, not bacteria. So antibiotics –medications for bacterial infections—won’t cure colds and flu. But even today, many people insist on using antibiotics to fight colds and flu.

The connection between germs and infection did not really develop until the late 1600s and the invention of the microscope. Even then, germs-infection association was very slowly recognized and –slowly—accepted. Other theories of disease dominated until the 20th century.

The earliest –and most enduring –explanation for disease was superstition: evil spirits, demons, curses, and punishment by the gods. Medical treatments often included magical rituals and remedies. Even as medical science evolved across cultures and over centuries, superstitious beliefs about illnesses persisted.

Medical skills were impressive in ancient Egypt, India, Mesopotamia, Persia, and China. But there was no knowledge of germs. The ancient Egyptians produced the earliest known text on diseases and treatments about 3000 BCE. And Egyptian medical practices, including dentistry, tumor removal, brain surgery and prosthetics, influenced many other cultures. Egyptian hospitals were possibly the earliest in the world. Medical remedies included ointments –along with magic potions, animal saliva, and magic incantations and prayers to the gods.

Medicine was also well developed in ancient India. In the Golden Age of Indian medicine, from about 800 BC to 1000 AD the famous Veda texts, detailing a wide range of conditions and remedies, were produced. India developed plastic surgery, anesthesia, dental care, and probably the earliest type of inoculation against smallpox. Hindus emphasized hygiene, so bathing and sanitary habits were part of medical care. They gave considerable attention to healthy diets and to plant-based medicines. But despite these great achievements, there was no understanding of germs.


The earliest theories of disease were the HUMORAL theory and the MIASMA theory. Both explanations survived into the 18th, even 19th century, and were only gradually replaced by the theory of germs as the cause of disease. We begin with the humoral theory.

Humoral theory was the basis of medical knowledge in ancient India, Greece, Persia, and Rome. According to this theory, Nature was structured on four elements: air, water, fire and earth. The humoral theory of disease was based on these elements. The four humors of the body were blood (the hot and wet like air); phlegm (cold and wet like water); yellow bile (hot and dry like fire); and black bile (cold and dry like earth). It was thought that Illness was caused by imbalances in these humors, just as imbalances in the elements caused disruptions in nature.
In India, the humoral theory became part of Hindu medicine around 300 BCE and continued for centuries. Various diseases were linked to specific imbalances of the humors – and, of course, evil spirits. The Persians and Greeks also had very advanced medical skills and hospitals, but imbalance of the four humors was the source of disease.

In Greece, Aristotle and Hippocrates rejected evil spirits and angry gods as the source of disease, but their anti-superstition argument was not universally accepted. And even those two great philosophers promoted the humoral theory of disease. However, they also argued for the emphasis on observation and evidence, the basis of empiricism.

Ancient Rome inherited and expanded on the medical knowledge of Greece, Persia and India. Its own major contribution was really sanitation and hygienic practices such as public baths. The most famous Roman physician, was Galen (161 CE), a Greek who moved to Rome, bringing major advances to the study of anatomy. Galen proved that the arteries carried blood, not air (as was the common belief). After the fall of Rome in the fifth century AD, Galen’s were preserved and translated from the Latin by scholars in the Islamic empire (roughly from 650-1260 CE), which had penetrated Europe. His texts became the established sources for medical study up to the Renaissance in the mid-1500s and beyond. The Islamic empire included non-Muslim minorities –Jews and Greeks, for example– who helped translate and transmit the knowledge of the ancients to undeveloped Europe. Although the humoral theory persisted, during the Islamic empire, medical knowledge accelerated. Free medical care and magnificent hospitals were established. And the science of chemistry emerged, gradually evolving from alchemy.
Some of the greatest physicians in Medieval times came from the Islamic empire. One was Al-Razi (865–925 CE), a Persian genius influenced by Hippocrates. He stressed the empirical approach to medical science, rejecting evil spirits. Al-Razi produced detailed explanations of surgical procedures and careful descriptions of diseases such as measles and smallpox that were dominant resources for centuries. Nevertheless, he attributed disease to imbalances in the humoral system.

Another great physician-scientist from the Islamic empire was Avicenna (Ibn Sina) (980-1037 AD). Known as the “father of modern medicine,” he wrote one of the most famous medical texts in history, the Canon of Medicine (1025), a kind of encyclopedia of anatomy, diseases and treatments. He realized that diseases might be passed from one person to another through the air –an early idea of contagion. This was truly radical. But Avicenna emphasized the humoral theory as the basis of disease and even for conditions such as stroke.


The miasma theory of disease was the belief that disease was created by foul air (miasma). Up to the 19th century China and numerous European cultures believed that diseases such as cholera, plague, malaria, and conditions such as stroke and heart attacks were all due to bad air, especially night air. It was easy to make this connection between sickness and putrid swamps, polluted water and rotting vegetation.

Medieval Christians in Europe considered disease basically a divine punishment, and depended largely on prayer and miracles for relief of illness. These remedies were supplemented with treatments such as the use of leeches to draw infected blood. Diseases were also attributed to foul air –the miasma theory.
There was no connection made between mosquitoes or other pests and diseases. There was no knowledge of contagion, despite the plain evidence during epidemics. Rather, the disease or condition was transmitted directly from foul air to the victim.

The Europeans were late to adapt hygiene. In fact, practices such as hand-washing and bathing were actually unpopular until the late 1800s and 1900s, although the importance of cleanliness was introduced first by the Romans and later by the Islamic empire.

European medicine developed when Greek and Islamic medical texts were translated into Latin. By the 16th century, there were advanced medical schools in Italy, especially Padua, where physician Andreas Vasalius and other medical pioneers produced scientific studies of human physiology and, subsequently, disease.

The growth of European cities over several centuries brought crowding and sanitation issues. Urban water systems and sewage structures were inadequate. The foul air encouraged continued support for the miasma theory, but scientists were beginning to understand germs and contagion.

In China, medical knowledge was much more advanced than European medicine. Chinese innovations such as acupuncture and many herbal medicines became famous and are still popular today. But the Chinese adhered to the miasma theory up to 19th century when the discovery and study of germs became widespread in Europe.


As science developed in the Renaissance, the late 1500s and 1600s, germ theory slowly -very slowly!– evolved. In 1546, the Italian scientist Girolamo Fracastoro was one of the first to introduce the new theory. Fracastoro was famous especially for my study of syphilis, which led to his theory that invisible “spores” transmitted disease through direct or indirect contact. This was really a radical argument: not every disease could be due to foul air.
The scientific development was made possible by the development of the microscope by Galileo and expansion of scientific thought in the Enlightenment period of the 17th and 18th centuries. The first person to actually view bacteria and protozoa under the microscope was the Dutch scientist, Antonie van Leeuwenhoek in the late 1670s.

The shift toward real science was occurring in other sciences like physics. Roger Bacon promoted objective scientific research and helped promote the decline of alchemy in favor of the science of chemistry. Isaac Newton rejected the theory of humors. But the fact that Newton established modern physics as he simultaneously practiced alchemy is a good example of the uneven emergence of real science.

By the 1800s, miasma theory was still prominent, but with growing scientific knowledge and equipment such as the microscope, knowledge of germs gradually developed. In 1854 the decisive shift to germ theory came with London’s Broad Street cholera epidemic. John Snow, a British physician-scientist examined the water supply for the Broad Street neighborhood. Dr. Snow realized that the cholera outbreak was due to contaminated water, especially water from one pump where many people filled their household water containers every day. But Snow’s brilliant insight occurred before the germs-disease link was really understood, and his argument was not widely accepted.

But over the next few decades, scientists discovered that specific germs caused specific diseases. In the 1870s the link was made between typhoid outbreaks and human waste. In 1876, Robert Koch discovered the bacterium that caused anthrax. This case was a final rejection of miasma and humoral theories. But even as knowledge of germs steadily increased, the miasma theory persisted. Even among prominent physician-scientists, the notion of contagion was controversial. Oh well!

The GERM THEORY took hold at last! This triumph was continued by the familiar work of Louis Pasteur and others.

Natl Geographic, March, 2020 Why Plague Doctors Wore Those Beaked Masks