Though the exact date is uncertain, early January 1665 fell within the period Cambridge suspended teaching due to bubonic plague, forcing Isaac Newton back to his family farm. There, isolated from university life, he developed calculus, optics theory, and the foundations of gravitational theory in roughly eighteen months.
The University of Cambridge shut its doors as plague deaths mounted in London and spread outward, and among the students sent home was an unremarkable 22-year-old named Isaac Newton, who had shown competence but no particular brilliance in his coursework. He returned to his mother's farm at Woolsthorpe Manor in Lincolnshire, ostensibly waiting out a public health crisis.
What happened next has become one of the most mythologized episodes in the history of science. Alone with his thoughts, Newton began working through problems of motion, light, and mathematics that had stumped him at university. He developed the method of fluxions—what we now call calculus—independently derived the binomial theorem, and began the optical experiments that would lead to his theory of light and color. The famous, possibly apocryphal apple is said to have fallen in the manor's orchard during this same period.
Newton himself later wrote that this was when he was 'in the prime of my age for invention' and minded mathematics 'more than at any time since.' The eighteen months at Woolsthorpe produced the intellectual scaffolding for Principia Mathematica, published over twenty years later. A pandemic that killed roughly 100,000 Londoners inadvertently gave the world modern physics.