Although the disease is less known outside tropical areas, in some cases travelers to regions where there is a risk of contracting it have become infected and brought the disease home with them.
१९७० से पहले केवल ९ देशों में महामारी फैली थी, लेकिन १९९५ तक यह संख्या बढ़कर ४१ हो गयी ।