by Giuseppe Scarlata, Maria Chiara Rosace | April 13, 2022.

Edited by Ivy Rose Sebastian.

As we enter the first quarter of the new year, the hypotheses about the current scenario of the pandemic are diverse. Virologists around the world are formulating conflicting theories: some say we are still far from the end; others think the virus is becoming endemic, mutating, and adapting increasingly to its host. Whatever the answer, the only certainty is that vaccines played a fundamental role in protecting the population and bringing us closer to normality (read our article: “COVID-19 and vaccines: can the transmission of the virus be stopped?”).

In recent years, technical terms such as “spill over,” “Real-Time PCR,” or “carrier” have become part of our colloquial language (read our article: “Molecular test: what does CT Value mean?”) In this new article, ScienceRely will talk about the origins of the term “carrier,” telling the story of the first carrier of Salmonella typhi: Mary Mallon, also known as Typhoid Mary.

Salmonella enterica subsp. enterica serovar typhi: the microorganism in her

Famously known for being the aetiological agent of Typhoid fever, Salmonella enterica serotype typhi is a gram-negative, rod-shaped, flagellated bacterium. The infection is contracted through the consumption of contaminated food or water and is prevalent in areas where there are poor hygiene conditions. The onset of the disease is rather deceptive, with symptoms such as haemorrhagic diarrhoea and fever with a characteristic “sawtooth” pattern: in the first week, the fever increases progressively, followed by a stable temperature of around 39°- 40°C in the second week and then it decreases in the third week. This febrile pattern, together with the aid of various laboratory diagnostic tests, facilitates the differential diagnosis with other pathogens, such as Brucella spp. Complications in the absence of adequate antibiotic treatment are intestinal perforation, hepatosplenomegaly, and pneumonia.

Prevention is clearly essential, so it is necessary to consume well-cooked food, well-sealed bottled water, and wash vegetables thoroughly. Isolation in the laboratory uses a culture medium called Agar SS (Agar Salmonella-Shigella) which helps differentiate it from bacteria of the genus Shigella. Due to the production of hydrogen sulphide, Salmonella colonies appear with a central black dot, in contrast to Shigella colonies, which appear colourless (Figure 1).

Figure 1. Isolation of Salmonella spp. on Agar SS (
Who was Mary Mallon?

Mary Mallon was born on 23rd of September 1869 in Cookstown, United Kingdom. In 1883, she decided to leave her small village to seek her fortune in the United States. She began working as a maid and cook for wealthy families, moving around frequently.

In the early 1900s, within the area between the cities of New York and Long Island, people belonging to the American nobility began to fall ill with typhoid fever. In 1906, six members of an eleven-member family, where Mary was working, became infected with typhoid and hired a sanitary engineer of the New York City Department of Health, George Soper, to investigate the cause of the infection. After careful investigation, Soper’s team concluded that contaminated water was probably the cause. Meanwhile, Mary Mallon continued to move around, working as a cook in several other homes.

The following year, in Manhattan, another family who Mary was employed under, died of typhoid fever. In theory, the disease was linked to poor hygiene conditions, and it seemed unusual that wealthy families were suffering from it. The case once again came to Soper’s attention and using information obtained from the employment agency, he was able to connect Mary’s presence to each of the twenty-two cases of typhoid fever that occurred in the areas of New York City and the Long Island.

Soper realized that Mary was in fact a carrier and that she was the root cause of the disease’s transmission. Despite her complaints, she was asked to collect stool and urine samples for microbiological investigation. She refused and fled but, the authorities caught up with her and placed her in forced isolation at Riverside Hospital. Around this time, her face debuted in the local newspapers and the press dubbed her as ‘Typhoid Mary’ (Figure 2).

Figure 2. Article on Mary Mallon published in the New York American newspaper, June 1909.

She remained in solitary confinement until 1910 and was then released with a promise to change jobs. For a short time, Mary kept her promise, but financial distress led her to change her name, and she moved in with other noble families under the name of Mary Brown. Four years later, following new cases of typhoid fever at Sloane Hospital for Women in New York City, Soper got back on her trail. As soon as she was located, Mary immediately surrendered to the authorities, and in 1915, she was taken back to Riverside Hospital. A stroke in 1932 kept her bedridden until her death on 11th November 1938.

Why didn’t Mary get sick and what does carrier mean?

A carrier is an individual who harbours pathogenic microorganisms without presenting any symptoms. This may be due to different molecular mechanisms, which may be linked to the immune system, or to genetic factors. In these cases, the micro-organisms live inside the host as saprophytes, i.e. they can benefit from the host in order to survive. The term is also used to identify individuals who are infected with transmissible pathogens but do not show any symptoms (‘asymptomatic’). Despite not being directly affected by the disease, the carrier is of great epidemiological importance, being an unwitting vector of pathogens. Mary Mallon was therefore a carrier: she had no symptoms of the infection but by eliminating the bacterium through the orofecal circuit, she transmitted it to those around her.

The risk in releasing the patient from Riverside Hospital is too great for this court to take. The damage that may be caused to innocent people […] is incalculable […] although the court has great compassion for this unfortunate woman, it must protect society from the possible danger of a further spread of contagion“.

(Judge Erlanger)

  1. Dougan G, Baker S. Salmonella enterica serovar Typhi and the pathogenesis of typhoid fever. Annu Rev Microbiol. 2014; 68:317-336.

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