Contagion: The issue of contagion was a massive one in urban areas, as sanitation was generally very poor in cities. Until the mid-nineteenth century, people knew little about how disease was spread, exacerbating the situation (people in the eighteenth century, for example, would not see anything wrong with digging a septic ditch near a public drinking well). The prevailing theory at the time was that disease was spread through miasma. That is, noxious air. The foul smell of disease and decaying flesh led many to believe that this is how disease was spread: through breathing this bad air. The miasma theory was only displaced by the germ theory of disease in the 1860s thanks to the research of scientists like John Snow and Louis Pasteur.
Cholera: During the nineteenth century, cholera killed tens of millions of people in Europe and its colonies. HIV did not become prominent until the late 1970s/early 1980s. While Plague, Measles, and smallpox were still issues during the nineteenth century, they did not match cholera is number of casualties, nor do they cause death by dehydration as cholera does.
Homeopathy: Homeopathy is a form of alternative medicine first introduced in the 1790s, which utilizes water in many of its "cures". Homeopathic remedies have been found ineffective by scientific research.
Louis Pasteur: Considered the founder of medical microbiology, French chemist Louis Pasteur made incredible breakthroughs in the causes and prevention of disease. His achievements include the first rabies vaccine, the reduction of deaths from puerperal fever, and the invention of a technique for sanitizing milk that came to be known as pasteurization. Advocated by Louis Pasteur, the germ theory of disease held that many deadly diseases (such as cholera) were spread by infectious microorganisms invading new hosts. The germ theory of disease displaced the miasma theory. The miasma theory was the belief that diseases such as cholera and the bubonic plague were spread through miasma, a form of "bad air". It is likely this theory took root due to the foul smell of the rotting bodies of the dead. The theory was displaced by the germ theory of disease in the nineteenth century.
John Snow: Skeptical of the miasma theory, John Snow began investigating possible causes of an outbreak of cholera on Broad Street in London. Using a dot map, he was able to localize a common factor among a majority of the cholera cases – the use of a water pump only three feet from an old cesspit. Snow's hypothesis that the outbreak had been caused by fecal matter leaking into the water supply was dismissed by the British government for being "too depressing".
Joseph Lister: Joseph Lister (1827-1912) is considered the pioneer of antiseptic surgery, applying the burgeoning science of microbiology to the practice of surgery. His sterilization of wounds and surgical instruments led to a disease of post-operative infections in surgery patients.
Macadam: Macadam is a type of road construction pioneered by Scottish engineer John McAdam in 1820. The method was the first to create hard-surface roads in the place of muddier dirt-top roads.