(Editor's note: This article originally appeared in the December 2005 issue of Scientific American magazine. We are posting it because of related news regarding swine flu.)

The 1918 pandemic flu virus is back, at least in the lab. After fishing out the virus's genes from preserved tissue samples, Jeffery Taubenberger of the Armed Forces Institute of Pathology and his colleagues report in the October 6, 2005, Nature that the virus was not a combination of human and avian flu strains, as was the case for other, milder pandemics.

Rather the 1918 virus probably leaped whole into the human population from another host. That means the H5N1 avian flu [at the time] raging throughout Asia and spreading to Europe might be able to do the same. In fact, it may be deadlier. In the October 7 Science, Terrence Tumpey of the Centers for Disease Control and Prevention described the recreation of the 1918 virus from the genome sequence. The revivified killer was lethal to mice initially infected with as few as 1,000 virus particles, whereas some H5N1 virus isolates have killed mice infected with just 10 or 15 particles.