apr 21, 1721 - The deadliest outbreak of smallpox in the history of Boston begins when the British ship HMS Sea Horse arrives in Boston Harbor with a crew of sailors who had survived a smallpox epidemic. One of the Seahorse crew who had cleared quarantine develops symptoms the next day and infects other people in a lodging house. Over the next 10 months, 5,759 cases of smallpox are recorded in Boston and 844 people die of the disease