Pedestrians walk along Michigan Avenue in Chicago this week. Bitter cold is hitting much of the Northern half of the U.S., with temperatures dipping into the single digits and below zero. The extremely frigid weather is expected to last for days.
Kamil Krzaczynski/Getty Images
A blood test developed by Harvard researchers checks for evidence of past infection with more than a thousand strains of virus, from about 200 virus families. The swine flu virus shown here, A/CA/4/09, rarely infects humans.
C. S. Goldsmith/CDC