Short answers, pulled from the story.
Arsenic earned the nickname "the poison of kings" because its symptoms are non-specific and it was used by ruling classes to murder one another before a reliable detection method existed. The Marsh test, developed in the 1830s, was the first sensitive chemical test capable of detecting its presence. In the Renaissance era arsenic was also called "the inheritance powder" due to its use in killing family members for a fortune.
Approximately 57 million people in the Bengal basin alone drink groundwater with arsenic above the World Health Organization's standard of 10 parts per billion. A 2017 report in Science found that more than 60 million people in Pakistan are exposed to arsenic-polluted drinking water. A study by IIT Kharagpur estimated that arsenic in groundwater across 20% of India's land potentially exposes more than 250 million people.
The primary industrial use of arsenic is in lead alloys, particularly to strengthen lead components in car batteries and in the production of ammunition. Gallium arsenide, an arsenic compound, is used in semiconductor integrated circuits that are faster than silicon-based circuits and can convert electrical energy directly into light in laser diodes and LEDs. Wood preservation with chromated copper arsenate was once the largest single industrial application in the United States, though a voluntary phaseout began on the 31st of December 2003.
In 2000, the US Food and Drug Administration approved arsenic trioxide for treating acute promyelocytic leukemia resistant to all-trans retinoic acid. Arsenicals such as melarsoprol are still used to treat trypanosomiasis because the disease is nearly uniformly fatal if untreated. A 2008 paper also reported success using arsenic-74 as a positron emitter to locate tumors in PET scans, producing clearer images than the previous agent iodine-124.
The Bradford sweet poisoning in 1858 was caused by the accidental adulteration of sweets with arsenic, resulting in 21 deaths. The incident was an early and prominent case of arsenic contamination in food and contributed to public awareness of arsenic's dangers in everyday goods.
Arsenic's toxicity comes from the affinity of arsenic(III) oxides for thiols, which are located at the active sites of many important enzymes. It disrupts ATP production by inhibiting lipoic acid in the citric acid cycle and uncouples oxidative phosphorylation, blocking mitochondrial respiration and ATP synthesis. The resulting multi-system organ failure is presumed to result from necrotic cell death, because energy reserves are too depleted for apoptosis to occur.