This 2003 electron microscope image made available by the Centers for Disease Control and Prevention shows mature, oval-shaped monkeypox virions, left, and spherical immature virions, right, obtained from a sample of human skin associated with the 2003 prairie dog outbreak. Monkeypox, a disease that rarely appears outside Africa, has been identified by European and American health authorities in recent days. (Cynthia S. Goldsmith, Russell Regner/CDC via AP)