Tourism is travel for recreation, leisure, religious, family or business purposes, usually for a limited duration. Tourism is commonly associated with international travel, but may also refer to travel to another place within the same country. The World Tourism Organization defines tourists as people "traveling to and staying in places outside their usual environment for not more than one consecutive year for leisure, business and other purposes". Tourism has become a popular global leisure activity. Tourism can be domestic or international, and international tourism has both incoming and outgoing implications on a country's balance of payments. Today, tourism is a major source of income for many countries, and affects the economy of both the source and host countries, in some cases being of vital importance. Tourism suffered as a result of a strong economic slowdown of the late-2000s recession, between the second half of 2008 and the end of 2009, and the outbreak of the H1N1 influenza virus. It then slowly recovered, with international tourist arrivals surpassing the milestone of 1 billion tourists globally for first time in history in 2012.