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In 1894 a young doctor working in Hong Kong during an outbreak isolated the bacterium that causes plague. This post tells Alexandre Yersin's story from the Historical Dictionary of Switzerland, a peer-reviewed biography and the World Health Organization, explains what plague is in terms your NCERT book uses, and works through how to read a case-fatality figure.
Who was Alexandre Yersin?
The Historical Dictionary of Switzerland records that Yersin was born on 22 September 1863 at La Vaux, near Aubonne, in the Swiss canton of Vaud, and died on 1 March 1943 at Nha Trang, in what is now Vietnam. His father died before he was born, and his mother supported the family by running a boarding school in Morges. He studied medicine in Marburg and then Paris, where he trained in microbiology and took his doctorate in 1888. He became a French citizen in 1889.
From 1889 he worked as an assistant to Émile Roux at the Institut Pasteur in Paris, and the two published three major papers on diphtheria in 1888, 1889 and 1890. A biography by B. J. Hawgood in the Journal of Medical Biography says that this work identified the toxin the diphtheria bacterium produces.
Why did he leave the laboratory for Asia?
Yersin did not stay in Paris. The Historical Dictionary notes several expeditions in Indochina from 1890, and Hawgood writes that he was the first European to explore and map the central highlands of Vietnam. He joined the French colonial health service, and in 1894 he was sent to Hong Kong to investigate an outbreak of bubonic plague.
What did he find in Hong Kong?
Hawgood's account is brief and clear: Yersin isolated the bacillus that causes plague from buboes, the swollen lymph nodes that give bubonic plague its name. The organism was later named Yersinia pestis in his honour. In 1895, back in Paris, he took part in developing an anti-plague serum.
He was not the only scientist studying that outbreak. The bacteriologist Kitasato Shibasaburo also worked on it, and the question of who should be credited caused a long dispute. A 1976 review in Bacteriological Reviews, Diagnosis of plague: an analysis of the Yersin-Kitasato controversy, was devoted to it.
What is plague, in today's terms?
The WHO fact sheet on plague says it is caused by Yersinia pestis, a bacterium usually found in small mammals and their fleas. People can be infected through the bite of infected fleas, through unprotected contact with infected body fluids or materials, or by breathing in particles from a patient with the pneumonic form. Bubonic plague, the most common form, has a case-fatality ratio of 30% to 60% if untreated. The WHO stresses that antibiotics are effective and that early diagnosis and treatment save lives.
Your NCERT Class 12 chapter Human Health and Disease lists plague among diseases caused by bacteria. Health questions belong with a doctor; this post is about history and reasoning.
Worked example: what does a 30% to 60% case-fatality ratio mean?
A case-fatality ratio is deaths among people who have the disease, divided by the number of people who have it. It is not the share of the whole population that dies. These are sample numbers for practice, not outbreak data.
Suppose a town of 50,000 people has 200 untreated cases of bubonic plague.
Lower end: 30% of 200 = 0.30 × 200 = 60 deaths.
Upper end: 60% of 200 = 0.60 × 200 = 120 deaths.
As a share of the whole town, even 120 deaths is 120 ÷ 50,000 = 0.0024, or 0.24%.
So "30% to 60%" describes how dangerous the disease is for someone who catches it and goes untreated. It says nothing by itself about how many people in a town will catch it.
What mistakes do students make with these numbers?
Using the wrong denominator. Case-fatality divides by cases, not by population.
Treating a range as one number. A range of 30% to 60% means outcomes vary; quote both ends.
Ignoring the word "untreated". The WHO figure is for untreated cases. Treatment changes the outcome.
If denominators keep catching you out, the Fermi Problem Library has a step-by-step solver that explains the reasoning, so you can see which number belongs on the bottom of each fraction. Plague also has an Indian chapter: our post on Waldemar Haffkine and the plague vaccine made in Bombay picks up the story in 1897.
How did Yersin spend the rest of his life?
According to the Historical Dictionary, he founded the first medical school in Hanoi in 1902 and the first Pasteur Institute in Nha Trang in 1905. Hawgood adds that he developed vaccines and antisera for people and animals, and as an agronomist introduced the rubber tree and the cinchona tree, the source of quinine, into the country. He lived in Nha Trang until his death.
What can a student take from how Yersin worked?
Learn the methods thoroughly first. His years with Roux in Paris gave him the skills he used alone in Hong Kong.
Go where the problem is. He found the plague bacterium by working in the middle of an outbreak.
Stay curious beyond your subject. Exploration, farming and medicine all fitted into one working life.
Next step: take any disease statistic from a news report and ask what its denominator is before you decide what it means.
