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    Oh Vapor So Foul!

    The man in this sketch is holding his nose to avoid breathing in the miasma. Miasma refers to a toxic vapor that people believed for centuries was a cause of many diseases, including cholera and plague. The idea that miasma causes disease was first proposed in the second century B.C.E. by a prominent Greek physician named Galen. He believed that miasma, which is identifiable by its foul smell, emanates from rotting organic matter and sickens people who live close enough to inhale it. Miasma was the predominant explanation for disease transmission from the time of Galen until the germ theory of disease became widely accepted in the late 1800s.

    A man holding his nose to avoid breathing in a miasma
    Figure \(\PageIndex{1}\): A man holding his nose to avoid breathing in a miasma. Image used with permission (CC BY 4.0, via Wikimedia Commons).

    What Is a Scientific Theory?



    Germ theory, which is described in detail below, is one of several scientific theories you will read about in human biology. A scientific theory is a broad explanation of events that is widely accepted by the scientific community. To become a theory, an explanation must be strongly supported by a great deal of evidence.

    People commonly use the word theory to describe a guess or hunch about how or why something happens. For example, you might say, "I think a woodchuck dug this hole in the ground, but it's just a theory." Using the word theory in this way is different from the way it is used in science. A scientific theory is not just a guess or hunch that may or may not be true. In science, a theory is an explanation that has a high likelihood of being correct because it is so well supported by evidence.

    Germ Theory: A Human Biology Example

    The germ theory of disease states that contagious diseases are caused by "germs," or microorganisms, which are organisms that are too small to be seen without magnification. Microorganisms that cause disease are called pathogens. Human pathogens include bacteria and viruses, among other microscopic entities. When pathogens invade humans or other living hosts, they grow, reproduce, and make their hosts sick. Diseases caused by germs are contagious because the microorganisms that cause them can spread from person to person.

    First Statement of Germ Theory

    Germ theory was first clearly stated by an Italian physician named Girolamo Fracastoro (pictured below) in the mid-1500s. Fracastoro proposed that contagious diseases are caused by transferable "seed-like entities," which we now call germs. According to Fracastoro, germs spread through populations, making many people sick, through direct or indirect contact between individuals.

    Girolamo Fracastoro
    Figure \(\PageIndex{2}\): Girolamo Fracastoro made the first clear statement of the germ theory of disease. (CC BY-NC 3.0; By Francesco Redenti, via Wikimedia Commons)

    Fracastoro's idea, though essentially correct, was disregarded by other physicians. Instead, Galen's idea of miasma remained the accepted explanation for the spread of disease for another 300 years. However, evidence for Fracastoro's idea accumulated during that time. Some of the earliest evidence was provided by the Dutch lens and microscope maker Anton van Leeuwenhoek, who discovered microorganisms. By the 1670s, van Leeuwenhoek had directly observed many different types of microorganisms, including bacteria.

    Evidence from Puerperal Fever

    One of the first physicians to demonstrate that a microorganism is the cause of a specific human disease was the Hungarian obstetrician Ignaz Semmelweis in the 1840s. The disease was puerperal fever, an often-fatal infection of the female reproductive organs. Puerperal fever is also called childbed fever because it usually affects women who have just given birth.

    Semmelweis observed that deaths from puerperal fever occurred much more often when women had been attended by doctors at his hospital than by midwives at home. Semmelweis also noticed that doctors often came directly from autopsies to the beds of women about to give birth. From his observations, Semmelweis inferred that puerperal fever was a contagious disease caused by some type of matter carried to pregnant patients on the hands of doctors from autopsied bodies. As a consequence, Semmelweis urged doctors and medical students at his hospital to wash their hands with chlorinated lime water before examining pregnant women. After this change, the hospital's death rate for women who had just given birth fell from 18 to 2 percent, which was a 90 percent reduction. Some of Semmelweis' findings are presented in Figure \(\PageIndex{3}\).

    Yearly mortality rates 1784-1849

    Figure \(\PageIndex{3}\): SemmelwWien Maternity Clinic (first vertical line) and decreased after doctors started disinfecting their hands (the second vertical line is showing how deaths from puerperal fever increased after doctors began doing autopsies). For comparison, he showed lower death rates at Dublin Maternity Hospital where autopsies were not performed. (CC BY-NC 3.0; By Power.corrupts via Wikimedia Commons)

    Semmelweis published his results, but they were derided by the medical profession. The idea that doctors themselves were the carriers of a fatal disease was taken as a personal affront by his fellow physicians. One of Semmelweis' peers protested indignantly that doctors are gentlemen and that gentlemen's hands are always clean. As a result of attitudes such as this, Semmelweis became the target of a vicious smear campaign. Eventually, Semmelweis had a mental breakdown and was committed to a mental hospital, where he died.

    Father of Germ Theory

    Tableau Louis Pasteur
    Figure \(\PageIndex{4}\): Louis Pasteur investigated the causes of diseases such as puerperal fever. (CC BY-NC 3.0; Albert Edelfelt , via Wikimedia Commons;)

    Throughout the later 1800s, more formal investigations were conducted on the relationship between germs and disease. Some of the most important were undertaken by Louis Pasteur (pictured in his lab in Figure \(\PageIndex{4}\)). Pasteur was a French chemist who did careful experiments to show that fermentation, food spoilage, and certain diseases are caused by microorganisms.

    He discovered the cause of puerperal fever in 1879. He determined it was an infection caused by the bacterium Streptococcus pyogenes, shown under magnification below. Although Pasteur was not the first person to propose germ theory, his investigations clearly supported it. He also became a strong proponent of the theory and managed to convince most of the scientific community of its validity. For these reasons, Pasteur is often regarded as the father of germ theory.

    Streptococcus pyogenes
    Figure \(\PageIndex{5}\): Pasteur discovered that the bacterium Streptococcus pyogenes causes puerperal fever. (CC BY-NC 3.0; By Photo Credit: Content Providers(s): [Public domain], via Wikimedia Commons)


    • A scientific theory is a broad explanation that is widely accepted because it is strongly supported by a great deal of evidence.
    • An example of a scientific theory is the germ theory of disease. According to this theory, contagious diseases are caused by "germs," or microorganisms.
    • The germ theory of disease was first proposed in the mid-1500s. It was not widely accepted until the late 1800s, when it was strongly supported by experimental evidence provided by Louis Pasteur.


    1. Define scientific theory.
    2. Contrast how the word theory is used in science and in everyday language.
    3. What is the germ theory of disease? How did it develop?
    4. Explain why Pasteur, rather than Fracastoro or Semmelweis, is called the father of germ theory.
    5. Galen and Fracastoro may have come up with different explanations for how a disease is spread, but what observations do you think they made that were similar?
    6. Use the explanation of Semmelweis’ research and the graph above in Figure \(\PageIndex{3}\) to answer the following questions.
      1. What was Semmelweis’ observation that led him to undertake this study? What question was he trying to answer?
      2. What was the hypothesis (i.e. proposed answer for a scientific question) that Semmelweis was testing?
      3. Why did Semmelweis track death rates from puerperal fever at Dublin Maternity Hospital where autopsies were not performed?
      4. What were two pieces of evidence shown in the graph that supported Semmelweis’ hypothesis?
      5. Why do you think it was important that Semmelweis compared Dublin Maternity Hospital and Wien Maternity Clinic over the same years?
    7. What is the difference between a microorganism and a pathogen?

    8. Explain why the development of the microscope lent support to the germ theory of disease.

    9. Does the observation of microorganisms alone conclusively prove that germ theory is correct? Why or why not?

    10. Who do you think was using more scientific reasoning - Semmelweis or the physicians that derided his results? Explain your answer.

    Explore More

    Watch this short video for a dramatization of Semmelweis' work on puerperal fever and its negative reception by other physicians at the time.

    How does a scientific theory differ from a scientific law? Watch this excellent TED animation to find out.


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