The US government keeps a stockpile of smallpox vaccines big enough to vaccinate every man, woman, and child in the country.
If you don’t know what smallpox is, be happy. Most of you reading this will probably never have to deal with it. Smallpox was a highly infectious disease that caused painful blisters to appear all over the body. Its mortality rate was about one third. Once a massive problem, it has since been dealt with and the last case of smallpox was diagnosed in 1977.
After the disease was eradicated in the US, smallpox vaccinations gradually stopped. Until relatively recently, the vaccine was only circulated to medical professionals for research. Following September 11th, 2001, however, the US government decided to prepare itself further for defense of terrorism- including disease warfare.
Theoretically, one could intentionally release the smallpox virus. The US government has prepared for this by updating and releasing a smallpox response plan, as well as manufacturing enough vaccines to vaccinate every person in the US.