Quarantine is the period during which a person or animal must be kept under observation in isolation from the community after having been in contact with an infectious disease. The duration of quarantine depends on the disease(s) concerned and their maximum incubation period. Quarantine procedures are now rarely used due to the reduced incidence of serious infectious diseases and the availability of vaccines for many of them. The term derives from the period of 40 days that ships from the Levant had to wait before their crews could disembark at medieval European ports, from fear of their carrying plague.