10 Deadly Agents the CDC Works With

A U.S. Marine from Camp Pendleton receives his smallpox vaccination before heading out for active duty in the Persian Gulf. © Tim Rue/Corbis

The 2014 discovery of misplaced variola rekindled fears over the possible impact of a new smallpox outbreak among a population no longer vaccinated against it. The U.S. maintains a strategic stockpile of smallpox vaccine which, if administered within three to four days of exposure, can blunt infection and vastly decrease fatalities. Still, bioterror concerns persist, especially if the virus could be genetically altered or otherwise weaponized [sources: Aleccia; CDC].

Smallpox exists in two types. Variola major involves a more prevalent rash and steeper fever, while the rarer, gentler variola minor kills in only 1 percent of cases. Variola major is subdivided into ordinary (90-plus percent of cases), modified (a mild version that crops up among the previously vaccinated), and two rare and severe kinds called flat-type (aka malignant) and hemorrhagic. While ordinary variola major kills around 30 percent of the infected, flat and hemorrhagic are nearly always fatal [sources: Inglesby et al.; CDC].

The sick pass the disease via lengthy face-to-face interaction, direct contact with infected bodily fluids or contaminated objects or, rarely, when the virus goes airborne in an enclosed setting like a bus or plane. After seven to 17 days of noncontagious incubation, victims experience two to four days of flulike symptoms, followed by four highly contagious days of skin and mouth rashes. Over the following weeks, these spread across the body and develop raised bumps, which fill with thick fluid and develop distinctive indented centers. Finally, pustules show up, gradually scabbing over, falling off and leaving behind pitted scars. Smallpox remains contagious until the final scab drops off [source: CDC].