The transmission of communicable diseases by international travelling constitutes a menace to countries by the spread or potential spread of diseases across frontiers. When travelling, persons may come in contact with diseases for which no measures of control have been taken and thus propagate them directly or indirectly to one or more countries. The more serious case, because less obvious, is when carriers become new reservoirs for animal vectors of endemic diseases, enabling their extension to countries where they are unknown or had already been eradicated. Since no public health measures exist in such cases, the diseases rapidly turn into epidemics causing loss of human life and a strain on health resources and infrastructure that must rapidly adapt to the new situation.
When Europeans travelled to the New World they brought with them many diseases unknown to the indigenous population which resulted in the wiping-out of entire villages. Wars fought in a different geographical and cultural backgrounds, pilgrimages (the massive annual migration to the Mecca in Islamic countries), migration, tourism and educational exchanges all, inevitably, contribute to a 'trade' of diseases.