Madame Tussaud's Mysterious Cause of Death Has Just Been Revealed

Previously it had only been documented that the 89-year-old wax sculptor died of "old age."

Marie Tussaud, the wax modeler behind one of London's most popular attractions, likely died of pneumonia, says a new study published in the journal Lancet Respiratory Medicine.

One of the first successful businesswomen of 19th century Europe and a pioneer of the cult of celebrity, Tussaud died in 1850 at the age of 89. Her death certificate only vaguely recorded "old age" as the cause of her demise.

According to her two sons, until a few days before her death, Madame Tussaud sat at the entrance of her exhibition - which now has branches in dozens of locations worldwide - to collect the public's shillings.

But this image of a strong, healthy woman in charge of her business until the very end is likely false.

"It was a family concern to depict a very efficient Madame Tussaud," first author Francesco Galassi, at the Institute of Evolutionary Medicine at the University of Zurich, Switzerland, said.

"However, a re-analysis of the correspondence of her youngest son Francis tells a different story," he added.

A letter written by Francis to his father in 1848, two years before Madame Tussaud's death, reveals that Marie was "growing very feeble."

"At times she is very ill and she suffers from asthma which allows her no rest at night ... Her legs are bad like yours, and she has bunions that hurt her when she walks," Francis wrote.

RELATED: Mysterious Disease in Iconic Wyeth Painting Diagnosed

According to Galassi and colleagues Louise Baker, archivist at Madame Tussauds, Roberta Ballestriero, at the University of the Arts, London, and Frank Rühli, at the Institute of Evolutionary Medicine at the University of Zurich, a cardiorespiratory disease can explain fatigue and weakness, asthma and varicose veins.

"Heart failure, primary or secondary to pulmonary or systemic disease (eg. hypertension) would account for all of these symptoms," the researchers wrote.

They noted that an alternative diagnosis could be progressive lung diseases including emphysema, chronic bronchitis, and asthma, which could still have had consequences on the heart.

Further help with the diagnosis came from historical sources, which report that Tussaud's final illness lasted five days.

"This is suggestive of an infection, such as pneumonia, which is still common today with patients with chronic obstructive pulmonary disease," Galassi said.

RELATED: Genetic Disorder Found in 15th Century Painting

Little is known about Marie Tussaud's life. Her memoirs are not of much help, since they appear to be a promotional effort for her museum rather than true accounts of her life.

Best known for modeling the fresh cut heads of the most famous victims of the French Revolution, Madame Tussaud was born Marie Grosholtz in Strasbourg, France, in 1761. She learned the craft of wax sculpturing from Phillipe Curtius, a physician and talented wax modeler. Marie and her mother moved to Paris with him and at his death in 1794 she inherited his entire collection of waxworks.

After marrying Francois Tussaud, Marie moved to the UK where she toured England, Ireland and Scotland for more than 30 years with her exhibition of waxworks. In 1835 she established her first permanent display in London's Baker Street with her famous likenesses, which included Voltaire, Benjamin Franklin and Napoleon.

Her self portrait, made in 1842 when she was age 81, is now on display at end of the exhibition.

Last week, the wax museum founded by Madam Tussaud announced that a new figure is in the works at the museum's London headquarters and will soon be added to its branches in Washington, D.C., New York and London.

Donald Trump's likeness will be in place by Inauguration Day on January 20, 2016.

WATCH: Why People Who Smoke Look Old