What was John Snow's critical contribution to public health?

John Snow's most significant contribution to public health came from his groundbreaking research on the transmission of cholera. By meticulously mapping the outbreak in Soho, London, during 1854, he was able to demonstrate a clear link between the spread of cholera and contaminated water sources. This was a radical departure from the prevailing miasma theory, which suggested that diseases were caused by "bad air." Snow’s application of statistical methods and geographical mapping to identify the source of the outbreak was pivotal in recognizing waterborne diseases and laid the groundwork for modern epidemiology and public health practices.

The other options do not accurately reflect Snow's contributions; he did not create a vaccine for cholera, invent any medical instruments, or advocate for fresh air as a cure for illnesses in a way that specifically relates to cholera. His work emphasized the importance of clean water and sanitation, which became fundamental principles in public health.