Vaccination is the most effective method of preventing infectious diseases. Widespread immunity due to vaccination is largely responsible for the worldwide eradication of smallpox and the restriction of diseases such as polio, measles, and tetanus from much of the world. The effectiveness of vaccination has been widely studied and verified including the influenza vaccine, the HPV vaccine, and the chickenpox vaccine. The World Health Organization (WHO) reports that licensed vaccines are currently available for twenty-five different preventable infections. Vaccines can be prophylactic such as to prevent or ameliorate the effects of a future infection by a natural or wild pathogen, or therapeutic such as vaccines against cancer are being investigated.