theory that bad air causes the spread of diseases. however, people started to find a correlation between water quality and waterborne diseases, which led to different water purification methods, such as sand filtering and chlorinating their drinking water. founders of microscopy, antonie van leeuwenhoek and robert hooke, used the newly invented microscope to observe for the first time small material particles that were suspended in the water, laying the groundwork for the future understanding of waterborne pathogens and waterborne diseases. in the 19th century, britain was the center for rapid urbanization, and as a result, many health and sanitation problems manifested, for example cholera outbreaks and pandemics. this resulted in britain playing a large role in the development for public health. before discovering the link between contaminated drinking water and diseases, such as cholera and other waterborne diseases, the miasma theory was used to justify the outbreaks of these illnesses. miasma theory is the theory that certain diseases and illnesses are the products of "bad airs". the investigations of the physician john snow in the united kingdom during the 1854 broad street cholera outbreak clarified the connections between waterborne diseases and polluted drinking water. although the germ theory of disease had not yet been developed, snow's observations led him to discount the prevailing miasma theory. his 1855 essay on the mode of communication of cholera conclusively demonstrated the role of the water supply in spreading the cholera epidemic in soho, with the use of a dot distribution map and statistical proof to illustrate the connection between the quality of the water source and cholera cases. during the 1854 epidemic, he collected and analyzed data establishing that people who drank water from contaminated sources such as the broad street pump died of cholera at much higher rates than those who got water elsewhere. his data convinced the local council to disable the water pump, which promptly ended the outbreak. edwin chadwick, in particular, played a key role in britain's sanitation movement, using the miasma theory to back up his plans for improving the sanitation situation in britain. although chadwick brought contributions to developing public health in the 19th century, it was john snow and william budd who introduced the idea that cholera was the consequence of contaminated water, presenting the idea that diseases could be transmitted through drinking water. people found that purifying and filtering their water improved the water quality and limited the cases of waterborne diseases. in the german town altona this finding was first illustrated by using a sand filtering system for its water supply. a nearby town that didn't use any filtering system for their water suffered from the outbreak while altona remained unaffected by the disease, providing evidence that the quality of water had something to do with the diseases. after this discovery, britain and the rest of europe took into account to filter their drinking water, as well as chlorinating them to fight off waterborne diseases like cholera.