The central character of the novel is an historian named Mark Easterbrook. The story comes together with Easterbrook as the protagonist; the story could not have worked with either Marple or Poirot. As it turns out, the murders have a more traditional cause - no spooks or ESP or anything supernatural. It's a not so simple case of poisoning. Thallium poisoning, to be precise.
Prior to this novel, I had never heard of thallium or thallium poisoning, although I've come to learn that prior to it being banned in the United States by Richard Nixon with a Presidential Executive Order in February 1972, thallium was widely used world wide as a poison for rodents. In the 1950's thallium found it's way into a number of mystery novels as a "poisoner's poison". In real life, also from the 1950's, there was an"Australian Thallium Craze" where several murderers used thallium as their poison of choice.
The Pale Horse is notable for having saved at least two lives after readers recognized the symptoms of thallium poisoning from its description in the book. In 1977, a 19-month-old infant from Qatar was suffering from a mysterious illness and brought to London. According to reports, a nurse by the name of Marsha Maitland recognized the symptoms of thallium poisoning from having read The Pale Horse and the child was saved. In a case from Latin America, a woman had saved the life of a neighbor, also after having read the novel. I've found two different versions of the Latin American story. Wikipedia says "In 1975, Christie received a letter from a woman in Latin America who recognized the symptoms of thallium poisoning, thus saving a woman from slow poisoning by her husband". However, another source says it was the wife who was poisoning the husband. Interesting, either way.
