Restricting movement of people to prevent the spread of disease has been used for thousands of years, long before the cause of infectious diseases was understood. The book of Leviticus in the Bible refers to restrictions for leprosy and the word quarantine derived from the 40 days used to try and prevent spread of the Black Death in the 14th century. Ships were routinely quarantined to prevent the spread of infections, including cholera. There is always a period of time between acquiring an infection and beginning to show symptoms. That period is known as the incubation period.