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1.4: Theories in Science

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    Oh Vapor So Foul!

    An individual 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 diseases was first proposed in the second century B.C.E. by a prominent Greek physician named Galen. They 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

    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 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.

    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 \(\PageIndex{3}\).

    Puerperal fever yearly mortality rates 1784-1849
    Figure \(\PageIndex{2}\): This graph compares mortality rates from 1784-1849 in two maternity clinics (Wien and Dublin). Rates increased in the Wien clinic when they started studying pathological anatomy in 1823. The rates decreased sharply in 1847 when they started using chlorine handwashing. The Dublin clinic, which did not study pathological anatomy, shows a low rate of infection throughout the timeframe.

    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.

    Discovering Microbes

    Throughout the later 1800s, more formal investigations were conducted on the relationship between germs and disease. Some of the most important was 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 and determined it was an infection caused by the bacterium Streptococcus pyogenes (Figure \(\PageIndex{5}\)). 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.

    painting of Louis Pasteur in his lab
    Streptococcus pyogenes
    Figure \(\PageIndex{3}\): Louis Pasteur (left) discovered that the bacterium Streptococcus pyogenes (right) causes puerperal fever.

    Emerging Diseases

    Scientific theories are not static and neither is the world around us. While we have been studying disease for hundreds of years, there is always more to learn. One reason for this is that organisms (such as those that cause disease) are always changing. This evolution of organisms leads to new diseases such as the COVID-19 pandemic. This resulted from a novel coronavirus (SARS-CoV-2) which is a descendent of coronaviruses that did not infect humans. Scientists are continually developing new strategies for learning about and curing emerging diseases.

    Microbiologist Kerry Pollard performing manual extraction of coronavirus
    Figure \(\PageIndex{4}\): Pennsylvania Commonwealth microbiologist Kerry Pollard performs a manual extraction of the coronavirus inside the extraction lab at the Pennsylvania Department of Health Bureau of Laboratories on Friday, March 6, 2020.


    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 in Figure \(\PageIndex{2}\) 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 the 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

    How is Semmelweis's work relevant to us today? Listen to this story from RadioLab to find out how his work gave us a powerful weapon against modern global pandemics. [Text transcript of Dispatch 2: Every Day is Ignaz Semmelweis Day]

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


    1. Man holding his nose by Wellcome Collection gallery, CC BY 4.0 via Wikimedia Commons
    2. Yearly mortality rates by Power.corrupts, public domain via Wikimedia Commons
    3. Louis Pasteur public domain via Wikimedia Common
      1. Streptococcus pyogenes by CDC, public domain via Wikimedia Commons
    4. Microbiologist by Governor Tom Wolf, licensed CC-BY 2.0 via Flickr
    5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0

    This page titled 1.4: Theories in Science is shared under a CK-12 license and was authored, remixed, and/or curated by Suzanne Wakim & Mandeep Grewal via source content that was edited to the style and standards of the LibreTexts platform.

    CK-12 Foundation
    CK-12 Foundation is licensed under CK-12 Curriculum Materials License