Susanne was boning up on 19th century field medicine at the College of Santa Fe's Fogelson Library, when she found a yellowed envelope tucked inside bearing the nightmarishly descriptive label "scabs from vaccination of W.B. Yarrington's children," signed and dated by the book's author more than a century ago. Despite the fact that this is exactly how 30 percent of all Tales From The Crypt episodes begin, she decided to open the envelope, which, as promised, contained a handful of dried-up old mummy scabs. Clearly, this was a book that no one but the author himself had ever read.
You see, back in the 1800s the vaccination process was a little more visceral, because it isn't Civil War medicine unless it's gross and disgusting. The pox on an infected person would eventually scab over and fall off. Doctors would then take those scabs and implant them in the skin of healthy people, inducing a mild case of smallpox that would run its course and then leave the patient immune. Vaccinations today still follow this same basic idea, except instead of shoving infected flesh into your body, doctors now take a tiny needle coated with the virus and stab you with it 15 times.
As opposed to the old cow-in-the-office method.
The library called the CDC, who sent the FBI to pick up the scabs for some reason (presumably in case the envelope was part of a trap set by al-Qaeda time lords).