The poliovirus is of special concern to public health because it can circulate for weeks without generating symptoms and hence can travel long distances, entering polio-free areas through land, sea, or air transport. The virus can also be transmitted via contaminated water and food. In addition to direct transmission from person to person, there are three other means by which the virus can spread: through contact with infected animals; via artificially manufactured products such as IV needles; and through windblown dust particles carried on people's shoes or clothes. Polio remains endemic in several countries around the world. India has the highest number of cases worldwide, with an estimated 1 million people affected annually. Other high-risk countries include Pakistan, Nigeria, Afghanistan, Cameroon, Ethiopia, Israel, Jordan, Lebanon, North Korea, Oman, Palestine, Syria, Tunisia, and Yemen. There have been no reported cases of wild poliovirus infection in Canada since 1999, but the vaccine-derived poliovirus continues to cause problems for immunized children around the world. In October 2000, an investigation conducted by Health Canada found that samples taken from two infants in Ontario had tested positive for the vaccine-derived poliovirus. Both babies were hospitalized, one for more than a month, and the other for four days.