During a smallpox outbreak, Cambridge, Massachusetts required adult residents to be vaccinated or pay a $5 fine under a state statute. Henning Jacobson refused and was fined; he argued the requirement violated his personal liberty under the Fourteenth Amendment. Justice Harlan's opinion for the Court articulated the classic formulation of the state 'police power': individual liberty is not absolute and must sometimes yield to reasonable regulations enacted for the common good, including compulsory vaccination during an epidemic. The Court demanded only that the measure have a real and substantial relation to public health and not be arbitrary or oppressive. Jacobson is the foundational American public-health case. It has been invoked in every subsequent generation — from the Spanish Flu, through mid-century quarantine and fluoridation cases, to modern disputes over school vaccination mandates and COVID-19 restrictions — and it remains the leading precedent balancing individual liberty against communal welfare under the general-welfare and police-power framework the Preamble anticipates.