"not tonight, britta. not tonight." ♪ 30 years ago eradicating smallpox was once of science's greatest achievements, described as one of the worst disease ever, it killed half a billion people in the last century alone. however, two countries -- russia and the u.s. -- still have hundreds of smallpox samples between them tucked away in secret laboratories. many countries believe these stockpiles should finally be destroyed. cnn's dr. sanjay gupta goes to the cdc which has one of these labs to get answers. >> reporter: smallpox is a disease no one has seen since the late 1970s.