Germ Theory of Disease

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Germ Theory of Disease


History and Scientific Foundations

Applications and Research

Impacts and Issues



The germ theory of disease states that microorganisms— organisms that, with only one known exception, are too small to be seen without the aid of a microscope—are the cause of many diseases. The microorganisms include bacteria, viruses, fungi, algae, and protozoa. The germ theory of disease also states that the microbes that cause a disease are capable of being recovered and will cause the same disease when introduced into another creature. This theory has withstood scientific scrutiny for centuries. Indeed, it is known with certainty that many diseases are caused by microorganisms. Two examples are anthrax, which is caused by the bacterium Bacillus anthracis, and bacterial meningitis, which is caused by Neisseria meningitidis.

While now an accepted part of infectious disease microbiology and the foundation of a variety of disciplines, such as hygiene and epidemiology (the study of the origin and spread of infections), the exact reasons why some microbes cause disease remain poorly understood and are still being investigated.

History and Scientific Foundations

Millenia ago, when microorganisms were unknown, some diseases were thought to be a consequence of divine punishment for a person's bad behavior. Illnesses that affected groups of people were sometimes attributed to the foul smelling gases from a nearby swamp or the vapors from sewage lagoon. While it is true that some microbes can become airborne and can cause disease when inhaled (anthrax is one example), this was not recognized for a long time. Other purported causes of disease included vapors created by the rotation of Earth or disturbances within Earth, which was thought to be hollow.

A publication dating back to 36 BC proposed that some illness was the result of the inhalation of tiny creatures present in the air. However, this farsighted view was the exception for centuries. With the development of the microscope in the seventeenth century by Robert Hooke (1635–1703) and Anton van Leeuwenhoek (1632–1723), it became possible to examine specimens, such as water, and to visually detect living organisms.

At that time, the prevailing view was that life and disease arose spontaneously from non-living material. Then, in 1668, the Italian scientist Francisco Redi (1627–1697) showed that maggots did not appear if decaying meat was kept in a sealed container, but that the maggots appeared if the meat was placed in the open air. This implied that the maggots were present in the air that contacted the meat, rather than spontaneously appearing on the meat.


ASEPSIS: Without germs, more specifically without microorganisms.

CARBOLIC ACID: An acidic compound that, when diluted with water, is used as an antiseptic and disinfectant.

COWPOX: Cowpox refers to a disease that is caused by the cowpox or catpox virus. The virus is a member of the orthopoxvirus family. Other viruses in this family include the smallpox and vaccinia viruses. Cowpox is a rare disease, and is mostly noteworthy as the basis of the formulation, over 200 years ago, of an injection by Edward Jenner that proved successful in curing smallpox.

EPIDEMIOLOGY: Epidemiology is the study of various factors that influence the occurrence, distribution, prevention, and control of disease, injury, and other health-related events in a defined human population. By the application of various analytical techniques including mathematical analysis of the data, the probable cause of an infectious outbreak can be pinpointed.

INFECTION CONTROL: Infection control refers to policies and procedures used to minimize the risk of spreading infections, especially in hospitals and health care facilities.

MICROORGANISM: Microorganisms are minute organisms. With the single yet-known exception of a bacterium that is large enough to be seen unaided, individual microorganisms are microscopic in size. To be seen, they must be magnified by an optical or electron microscope. The most common types of microorganisms are viruses, bacteria, blue-green bacteria, some algae, some fungi, yeasts, and protozoans.

PUERPERAL FEVER: Puerperal fever is a bacterial infection present in the blood (septicemia) that follows childbirth. The Latin word puer, meaning boy or child, is the root of this term. Puerperal fever was much more common before the advent of modern aseptic practices, but infections still occur. Louis Pasteur showed that puerperal fever is most often caused by Streptococcus bacteria, which is now treated with antibiotics.

SPONTANEOUS GENERATION: Also known as abiogenesis; the incorrect discarded assumption that living things can be generated from nonliving things.

VACCINATION: Vaccination is the inoculation, or use of vaccines, to prevent specific diseases within humans and animals by producing immunity to such diseases. The introduction of weakened or dead viruses or microorganisms into the body to create immunity by the production of specific antibodies.

Early in the eighteenth century, it was observed that people could be protected from developing smallpox by exposing them to pus from the lesions of other people with the illness. While we now recognize this as the basis of vaccination, at the time the idea—that something in the illness could protect others from the malady—was revolutionary. The English physician Edward Jenner (1749–1823) is recognized as the founder of the practice of vaccination. Jenner noticed that dairy workers who had been exposed to cowpox, a milder disease similar to smallpox, seldom contracted smallpox. He showed that injecting people with fluid from the cowpox blisters (which was subsequently shown to contain the cowpox virus, which is related to the smallpox virus) conferred protection against smallpox.

In 1848, Hungarian physician Ignaz Semmelweis (1818–1865) discovered that a disease called puerperal fever could be spread from corpses to living patients by attendants who did not wash their hands between the autopsy room and the hospital ward. Handwashing greatly reduced the number of these infections. In 1854, English physician John Snow (1813–1858) demonstrated that an ongoing cholera epidemic in London was caused by water coming from a particular pump. When the water flow from the pump was shut off, the outbreak ended.

However, even with the accumulating weight of evidence that some agent was responsible for various diseases, many physicians continued to maintain that these agents did not exist because they could not be seen with the unaided eye. If they did not exist, then they could not be the cause of disease. It remained for Agostino Bassi (1773–1856), Louis Pasteur (1822– 1895), and Robert Koch (1843–1910) to perform the research necessary to finally convince the scientific community that germs did, indeed, cause disease.

In 1835, Bassi proposed the germ theory for the first time, when he hypothesized that a lethal disease of silkworms was due to a microscopic living organism. The agent was subsequently shown to be a fungus that was named Beauveria bassiana. Then, in a series of experiments in the middle of the nineteenth century, Pasteur convincingly demonstrated that the spoilage of wine, beer, and foods were caused by something in the air and not by the air itself.

In 1875, concrete evidence for the germ theory was provided by Robert Koch, who showed that Bacillus anthracis was the cause of anthrax in cattle and sheep.

Koch's step by step approach to his experiments laid the foundation for a series of conditions that must be met to demonstrate that a particular microorganism is the cause of a particular disease. The following conditions came to be known as Koch's postulates.

Koch's postulates drove the nail into the coffin of the theory of spontaneous generation. Once scientists accepted that the germ theory of disease was valid and began to search for more examples of microbial-caused diseases, the floodgates opened. By the end of the nineteenth century, it had been established that microbes were responsible for cholera, typhoid fever, diphtheria, pneumonia, tetanus, meningitis, and gonorrhea, as a few examples.

Also in the nineteenth century, English physician Joseph Lister (1827–1912) demonstrated that the development of infections in patients following surgery could be drastically reduced if a spray of carbolic acid was applied over the wound during surgery and surgical dressing put on the wound was soaked in the chemical. Since carbolic acid was known to kill microbes present in sewage, Lister helped convince people that microorganisms were important in post-operative infections.


Robert Koch is considered to be one of the founders of the field of bacteriology. He pioneered principles and techniques in studying bacteria and discovered the specific agents that cause tuberculosis, cholera, and anthrax. For this he is also regarded as a pioneer of public health, aiding legislation and changing prevailing attitudes about hygiene to prevent the spread of various infectious diseases. For his work on tuberculosis, he was awarded the Nobel Prize in 1905.

Koch's postulates

  • The particular microorganism must be present in every case of the disease.
  • That microorganism must be able to be isolated from a person or other creature host with the particular disease and must be capable of being grown in a pure form free from other organisms. (This condition has since been modified, since not all organisms can be grown in the laboratory. However, with molecular techniques of organism identification that are based on the detection of certain unique sequences of genetic material, the microbe does not always need to be grown to fulfill this condition.)
  • The microorganism that is recovered from the pure culture is capable of causing the disease when introduced into a previously healthy test creature.
  • The microorganism can be recovered from the infected creature and can be shown to be the same as the originally recovered or detected microbe.

Applications and Research

The germ theory is applied to infection control in hospitals, the treatment of food and water, and efforts to control the spread of infection in natural settings. Examples of the latter are the various vaccination and disease prevention programs that are spearheaded by agencies such as the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC). Even in the present day, research continues to identify the microbes responsible for diseases, to rapidly and accurately detect their presence, and to devise strategies that will minimize or completely prevent the particular diseases.

Impacts and Issues

The germ theory is profoundly important in understanding and preventing a variety of diseases. Knowledge that microorganisms can cause disease spawned efforts to prevent the microbes from coming into contact with people, food, water, and other materials. The practices of disinfection, sterilization, personal hygiene, and proper food preparation have their basis in germ theory.

Knowledge that many diseases are caused by microorganisms, and that the microbes can be spread from person-to-person and from an inanimate surface to a person spurred the development of techniques to minimize or prevent microbial spread. One example is asepsis—the treatment of living and non-living surfaces to kill or prevent the growth of associated microorganisms. Aseptic technique is one of the cornerstones of research microbiology and is crucially important in medicine. Up until the middle of the nineteenth century, the absence of aseptic techniques during operations made surgery a risky procedure. However, after the adoption of techniques to minimize microbial contamination of wounds and the airborne spread of microorganisms, the mortality rate following surgery plummeted. The infection control practices that are routine in hospitals today are a result of the germ theory.

Similarly, knowledge that some disease-causing bacteria, viruses, and protozoa—particularly those that normally reside in their intestinal tract—can be spread via the contamination of water by feces prompted the implementation of techniques of water treatment. Techniques of drinking water treatment that include filtration, chlorination, or exposure of the water to ozone or ultra-violet light are designed to kill potentially harmful microbes in the water.

The techniques of modern day molecular biology have an important place in germ theory. Detection and identification of microorganisms based on the presence of target sequences of genetic material is making infection control more rapid and efficient. Furthermore, the use of antibodies and other compounds to block the adherence of microbes to living and non-living surfaces is useful in minimizing the spread of infections.

The discipline of epidemiology is rooted in the germ theory. Epidemiology is essentially the germ theory in reverse. Rather than tracing the path from the source of a microbe to the disease, an epidemiologist begins with a disease and then, by various means, determines the source and geographical dissemination of that particular disease. For example, a 2006 outbreak of disease that occurred in several Midwestern states in the United States was traced to a crop of organic spinach contaminated with the bacterium Escherichia coli O157:H7. Epidemiology is also important in designing strategies to combat an ongoing disease outbreak and in minimizing the chances of future illnesses.

Strategies to minimize the spread of disease-causing microorganisms are often wise. However, concern with the potential for microbial safety in the home and work-place has fostered a sense of urgency that is out of proportion to the risk posed by the microbes. Supermarket shelves are lined with antibacterial products designed to keep a home almost free of microbes. While this may seem sensible, it has, in fact, spawned the development of increased resistance of some microbes to the chemicals being used to control or kill them. In addition, evidence is accumulating that the human immune system requires exposure to microorganisms to keep the system primed and capable of a rapid and efficient response. The strategy of disinfecting a house may be contributing to an increase in allergic diseases, since the immune system may over-react when confronted by a foreign substance, such as a microorganism.

See AlsoBloodborne Pathogens; Disinfection; Koch's Postulates.



Ewald, Paul. Plague Time: The New Germ Theory of Disease. New York: Anchor, 2002.

Tierno, Philip M. The Secret Life of Germs: What They Are, Why We Need Them, and How We Can Protect Ourselves Against Them. New York: Atria, 2004.

Waller, John. The Discovery of the Germ: Twenty Years That Transformed the Way We Think About Disease. New York: Columbia University Press, 2003.

Brian Hoyle