Incidence measures how many new cases of a disease occur in a population over a specific period, while prevalence measures the total number of existing cases at a given point in time -- a chronic disease can have low incidence but high prevalence if people live with it for years. John Snow's 1854 investigation of a cholera outbreak in London's Broad Street area, in which he mapped cases and traced them to a single contaminated water pump, is widely credited as the founding event of modern epidemiology because it demonstrated that disease transmission could be understood and controlled through systematic observation before germ theory was even accepted. A crude rate (such as a crude death rate) describes an entire population without adjustment, while an age-adjusted rate statistically controls for differences in age structure between populations, which matters because a population with more elderly people will naturally show a higher crude death rate even if underlying health is identical. These distinctions matter in practice: comparing two countries' raw mortality rates without age-adjustment can lead to the wrong conclusion about which population is actually less healthy.