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• Smallpox
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A vaccine is a substance made of weakened or killed disease germs designed to make a body immune to (safe against) that particular infectious disease. Effective vaccines change the immune system (the body's natural defense system against disease and infection) so it acts as if it has already developed a disease. The vaccine prepares the immune system and its antibodies (disease-fighting chemicals) to react quickly and effectively when threatened by disease in the future. The development of vaccines against diseases ranging from polio to smallpox is considered among the great accomplishments of medical science.

Smallpox

The first effective vaccine was developed against...