Typhoid Mary

How one asymptomatic woman spread typhoid to dozens of people and raised a host of bioethical questions.

Mary Mallon was born in Cookstown, County Tyrone in Ireland in 1869. She emigrated to New York City when she was 15 and worked her way up through the servant ranks to the highly respectable position of cook. Over the years she ran the kitchens & cooked for various families around the city. In the summer of 1906 she was the cook for the Warren family (Charles Warren, banker to the Vanderbilts) as they vacationed in a rental house in the very upscale Oyster Bay, Long Island.

Over the course of that summer, 6 members of the household got sick with typhoid. No one else in Oyster Bay contracted the disease, a disease typically associated with the poor. Concerned for the reputation of the rental house, the owner of the home knew the source of the typhoid had to be found or it would be difficult to ever rent the home again. George Soper, a freelance civil engineer, was hired to investigate the source and he traced it back to the Warren family’s former cook, Mary Mallon.

Tenement housing in New York provided ideal conditions for the spread of diseases including typhoid.


Typhoid, or more formally typhoid fever, is a form of salmonella (a bacteria) that can spread through tainted water or food that has come into contact with fecal matter. You find it in places with poor hygiene and poor sanitation, which is why it is generally associated with the poor.

New York City in the early 20th century was a much dirtier place than today. The population of the city was doubling every decade. The tenement housing of Manhattan’s Lower East Side was an overcrowded jungle of people and it was common for a family of 10 to live in a 325 square foot apartment. Add to the mix the 150,000 – 200,000 horses of the city, each of which created about 25 pounds of manure a day. It all led to filthy conditions that were ideal for typhoid and other bacterial diseases.

Mary, seen in the first bed, during her first quarantine at North Brother Island.

Forced Quarantine

Soper tracked down Mary and he documented a trail of typhoid in her wake. Over 10 years Mary worked for 8 different New York families, 6 of those families contracted typhoid and 1 person died. Despite this evidence Mary was adamant that she never had typhoid and she never felt sick. She was partially right.

It turned out that she was a “healthy carrier” of typhoid, someone who had the disease but never really felt sick. She was asymptomatic and went about her life unaware that she even had the disease, let alone that she was spreading it to other people (not unlike asymptomatic carriers of COVID-19).

Eventually she was forced against her will into quarantine by the New York City Health Department. In 1907 she was sent to North Brother Island in the East River which was being used as a quarantine center for people sick with infectious diseases. She remained there for 3 years, during which time her story of forced quarantine made it into the papers where she was dubbed “Typhoid Mary”.

In 1910 she was released from quarantine on the condition that she would never work as a cook again, since she had most likely transmitted typhoid through the food she prepared. She kept to this agreement for a while, working as a laundress, but eventually she disappeared from public health officials and started work as a cook again under assumed names. The pay and working conditions of a laundress were far below that of a cook for a wealthy family. She was caught working at Sloane Hospital for Women after an outbreak of typhoid infected 25 people and killed 2. She was sent back to North Brother Island where she lived until she died in 1938 at the age of 69 (still carrying typhoid).

Typhoid Mary

Mary Mallon’s legacy is one of bioethical questions. In the early 20th century the science of communicable diseases was in its infancy, and Mary’s suspicion of the New York Health Department was not unusual. She felt fine, so how could she be carrying/spreading a deadly disease?

Her quarantining raises ethical questions of how far the government should go to protect the general public. When weighing an individual’s civil liberties against the health of the public which is greater? Despite never being convicted of a crime she was imprisoned on North Brother Island for the safety of the public. Was it more ethical to quarantine her the first time or the second time, or at all? Knowing that other people were also asymptomatic carriers of typhoid why was she kept in isolation for nearly 30 years while others walked free? As a healthy carrier she was an unlucky innocent victim of a disease, but she also chose to go back to cooking which she knew might endanger lives. The questions raised by Typhoid Mary are still relevant today.

Added item: There is a good hour-long documentary by PBS, The Most Dangerous Woman in America, on the story of Mary Mallon. You can also find a bootleg copy of the documentary on YouTube:

John Snow’s Cholera Map

Through his medical investigation, Dr. John Snow helped solve how cholera is spread and created a legendary data visualization in the process.

With the Industrial Revolution, London’s population grew enormously. People from the countryside moved to the city for work and for a different life. London became the largest city on Earth. Between 1750 and 1850 it’s estimated that London’s population doubled, from around 1 million to around 2.3 million people. What grew with it was a civil engineering crisis in how to handle so many people in such close quarters. In short: what to do with the filth? By 1850 modern plumbing had not been extended to all parts of the city and specifically the Soho area. People had cesspools in their basements where they would empty their waste. In other places the sewage was emptied into the River Thames, which was also a source of drinking water.

London’s booming population growth in the early 19th century led to filthy conditions.

Modern germ theory states that microscopic organisms are responsible for the spread of disease. Before we understood this people believed in the miasma theory which claimed that disease was spread by “bad air”. For centuries people believed that epidemics were being spread by dirty air, they had no knowledge of microorganisms. It’s not entirely misguided. Things that smell bad frequently do, in fact, have disease. So while “bad air” may be a warning sign that disease may be present, it’s not the air itself that causes disease. In mid-19th century London miasma theory was the prevailing scientific theory but some scientists were beginning to doubt its validity.

Dr. John Snow challenged the prevailing miasma theory of disease through research and data.

You Know Something John Snow

Cholera is spread through tainted water or food that has come into contact with fecal matter. Between 1846 to 1860 the world was in a cholera pandemic, and in 1854 there was an outbreak in the Soho district of London. Nobody knew exactly how cholera spread but Dr. John Snow had a theory that it wasn’t miasma. A few years earlier in 1849 he published On the Mode of Communication of Cholera where he laid out a theory that a germ (that had yet to be identified) was responsible for cholera. He believed that cholera was spread by “…the emptying of sewers into the drinking water of the community.” The 1854 outbreak in Soho gave him a chance to prove his theory.

In the first 7 days of the outbreak 10% of the neighborhood died. Like a medical detective Snow began investigating the addresses of the deaths. He spoke to residents of the area, he asked where they got their water from, he took down notes, he looked at the sources of water for that part of London. The thing that was truly groundbreaking was that he visualized his data. He drew a map of the area, he noted the locations of water sources, and he added black bars at the addresses where deaths had occurred.

A detail of Snow’s 1854 cholera map. The Broad Street pump is at the center as a circle, and the deaths per address are the stacked black bars. You can view the full map here.

Unlike a data table, a data visualization has the ability to quickly & easily show trends. With a glance you can see patterns or outliers. You can tell a visual story with numbers. As Snow’s visualization grew he could see that cholera deaths clustered by one water source in particular: the Broad Street pump. He was able to show that other addresses in the area, who had their own private water sources (such as a local workhouse and a brewery) were mostly spared. The workhouse had 18 deaths but all of those individuals had separately gone to drink water from the Broad Street pump. This helped disprove the miasma theory because all of the workers should have gotten sick by the same “bad air”, but they didn’t. He took his findings to the local authorities. They found that the Broad Street pump was near a cholera infected home whose cesspool was leaking into the surrounding soil and infecting the water supply. Authorities removed the handle to the pump and deaths decreased.

Snow’s cholera map helped create modern epidemiology. COVID-19 visualizations are directly influenced by Snow’s work.

the Visualization of Data

To say that John Snow’s cholera map is legendary is not an exaggeration. Anyone with a passing knowledge of data visualization knows about his map. Modern epidemiologists still talk about his work. Snow’s methodical approach to data collection & data visualization influenced public policy and helped London prepare for the next cholera outbreak. It helped disprove miasma theory and advanced the modern germ theory we still use today. His cholera map helped make John Snow the father of modern epidemiology.

You can see the evolution of Snow’s work in today’s COVID-19 reporting. Contact tracing, the mapping of infections, accounting for local public policies regarding masks, tracking superspreader events – it’s all influenced by Snow’s 1854 cholera map.

Added info: Today there is a replica of the water pump where the old one stood, but Broad Street is now called Broadwick Street. The pump sits just outside of the John Snow pub.

Playing off of the lead character Jon Snow’s name, a White Walker from Game of Thrones stood outside of the John Snow pub in Soho in 2014. Photo via reddit.