The nation of Guinea has officially been declared Ebola-free by the World Health Organization. It’s a major milestone in the fight against the virus that killed more than 11,000 around the world—2,500 in Guinea alone, where doctors say the outbreak first began. As the BBC notes, being free from the virus means that at least 42 days have passed—the length of two incubation periods for the virus in humans—since the last infected person received two negative test results.