Although most countries no longer produce smallpox vaccine, some countries still stockpile smallpox vaccine. Smallpox is an infectious disease caused by the smallpox virus, which is mainly transmitted through the respiratory tract and contact, and initially manifests itself in severe flu-like symptoms, followed by a rash. The cowpox vaccine is an effective vaccine against smallpox, invented by British doctor Jenner and made from cowpox virus. The cowpox vaccine helps the human body to develop immunity against the smallpox virus and curb the attack of smallpox virus to a greater extent. Although the World Health Organization has declared that smallpox is extinct on earth and the chance of humans contracting smallpox is very small, some countries still produce small quantities of smallpox vaccine for stockpiling.