Classification and Patterns of Disease
Senior 4 · Biology
Classification and Patterns of Disease
Lesson 1 of 5
25 min
In September 1854, hundreds of people living around Broad Street in London died of cholera within a few days. The doctor John Snow did not look for bad smells, as most doctors of his time would have done. He went from house to house, asked where each family drew its water, and marked every death on a street map. The deaths clustered around one public pump. Snow persuaded the parish authorities to take the handle off it, and the outbreak, already slowing, ended. Snow never saw the cause: the bacterium was not proved to cause cholera until Robert Koch's work 30 years later.
In S1 you sorted diseases into infectious and non-infectious, and in S2 you met pathogens and the immune system. This lesson asks a different question: how did people decide what causes a disease, and what counts as proof?
Definition
Disease: a disorder of structure or function in an organism that produces specific signs or symptoms and is not simply the result of an injury.
Aetiology (etiology): the cause, or the study of the causes, of a disease.
Pathogen: a microorganism or virus that causes disease, for example Vibrio cholerae, the bacterium that causes cholera.
Miasma theory: the old idea that diseases come from "bad air" given off by rotting matter, sewage or swamps.
Germ theory of disease: the theory that many diseases are caused by specific microorganisms that invade the body and multiply in it.
Koch's postulates: four criteria, published by Robert Koch in the 1880s, used to show that a particular microbe causes a particular disease.
Every society has needed an explanation for illness, and each explanation led to its own kind of treatment.
| Theory | What it said causes disease | Treatment it led to | Why it was abandoned |
|---|---|---|---|
| Supernatural | Spirits, curses, sin or the anger of gods | Prayers, offerings, charms | Could not predict who falls ill; the same disease follows the same pattern in believers and non-believers |
| Humoral (Hippocrates, about 400 BCE; Galen, about 160 CE) | An imbalance of four body fluids: blood, phlegm, yellow bile and black bile | Bleeding, purging, special diets | Dissection and physiology found no such balance; bleeding often made patients worse |
| Miasma | "Bad air" from rotting matter, sewage and swamps | Cleaning streets, draining swamps, burning fires to clear the air | Snow showed cholera followed the water supply, not the air; malaria was found to be carried by mosquitoes |
| Spontaneous generation | Living things, including the "animalcules" in rotting food, arise from non-living matter | None directly: it meant microbes were thought to be a result of decay, not a cause | Pasteur's swan-neck flasks (about 1859 to 1862): boiled broth stayed sterile until dust from the air could reach it |
Notice that a wrong theory can still lead to useful action. Clearing sewage to remove "bad air" also removed the faeces that spread cholera and typhoid, so death rates fell for the wrong reason.
In 1546 the Italian physician Girolamo Fracastoro suggested that tiny "seeds" could pass a disease from person to person by contact, on clothing or through the air. Nobody could see such seeds, and the idea faded. Three centuries later, a run of careful observations brought it back:
Worked example 1: Snow's "grand experiment"
In parts of south London in 1854, two water companies laid pipes down the same streets. The Southwark and Vauxhall Company drew water from the Thames below London's sewage outfalls; the Lambeth Company had moved its intake upstream, above them. Snow counted cholera deaths in the first seven weeks of the epidemic. His figures (Snow, 1855) were:
| Water company | Houses supplied | Cholera deaths |
|---|---|---|
| Southwark and Vauxhall | 40 046 | 1 263 |
| Lambeth | 26 107 | 98 |
(a) Calculate the deaths per 10 000 houses for each company. (b) How many times greater was the risk in Southwark and Vauxhall houses? (c) Explain why this comparison is strong evidence against the miasma theory.
Showing that a microbe is present in sick people is not enough: it could be a harmless passenger, or a result of the disease rather than its cause. Koch's postulates set a higher standard.
Koch's postulates are a powerful test, but they cannot be met for every infectious disease:
Today scientists add molecular evidence: for example, finding the pathogen's genes in diseased tissue, or showing that removing one gene makes the microbe harmless (the "molecular Koch's postulates").
Worked example 2: a doctor who infected himself
Until the 1980s, most doctors believed that stomach ulcers were caused by stress and acid. In 1982 the Australian researchers Robin Warren and Barry Marshall found a curved bacterium, now called Helicobacter pylori, in the stomach lining of almost every patient with gastritis or an ulcer, and grew it in culture. Marshall then swallowed a culture of the bacterium himself, developed gastritis within days, and the bacterium was recovered from his own stomach. They received the 2005 Nobel Prize in Physiology or Medicine. Which of Koch's postulates did each step satisfy, and why was Marshall's step unusual?
Germ theory explains infectious diseases, but it cannot explain why only some exposed people fall ill, or what causes diabetes, sickle-cell anaemia or depression. Modern medicine therefore explains disease as the result of several interacting causes. For infections, epidemiologists use the epidemiological triangle: disease appears where a susceptible host, an agent (the pathogen) and a suitable environment come together.
The triangle explains why public health works on all three corners. Against cholera, for example: clean piped water and latrines change the environment; oral cholera vaccine makes the host less susceptible; chlorinating water kills the agent. Breaking any one side can stop transmission. For non-infectious diseases the "agent" is replaced by risk factors such as tobacco, a salty diet, physical inactivity or an inherited allele.
TED-Ed: "How a few scientists transformed the way we think about disease" (Tien Nguyen). Watch for the evidence that overturned the idea that disease spreads through clouds of bad air.
Warning
Common mistakes
Tip
Memory hook. Koch's postulates in four words: Found, Grown, Given, Found again. The microbe is found in every case, grown in pure culture, given to a healthy host (who gets the disease), and found again in that host.
Practice: theories of disease
Key points
Sources and credits
Quick check
Put Koch's postulates in the order they are carried out.
Tap the cards in the right order.
