A rogues gallery of the viruses (left to right) that cause MERS, SARS, and influenza. Niaid; 3D4Medical; Niaid/Science Source hide caption

itoggle caption Niaid; 3D4Medical; Niaid/Science Source

Joseph Robert gets a flu shot at the Metropolitan Multi-Service Center in Houston during the Falls and Flu Prevention Day, sponsored by the National Council on Aging and Sanofi Pasteur, on Sept. 26, 2013. Aaron M. Sprecher/AP Images for National Council on Aging and Sanofi Pasteur hide caption

itoggle caption Aaron M. Sprecher/AP Images for National Council on Aging and Sanofi Pasteur

New research finds a close connection between the flu that devastated the horse population in North America in the 1870s and the avian flu of that period. Hulton Archive/Getty Images hide caption

itoggle caption Hulton Archive/Getty Images

The fall armyworm, a corn pest, is now also a vaccine factory. Wikimedia Commons hide caption

itoggle caption Wikimedia Commons

Dr. Tom Nary is the director of health services at Boston College. Tovia Smith/NPR hide caption

itoggle caption Tovia Smith/NPR

Physician assistants Scott Fillman (left) and Andrew Hunadi get ready to see patients with flu symptoms, in a tent erected just outside the emergency entrance at the Lehigh Valley Hospital in Allentown, Pa. Matt Rourke/AP hide caption

itoggle caption Matt Rourke/AP

Four-year-old Gabriella Diaz gets a flu shot at the Whittier Street Health Center in Boston, Mass., on Wednesday, the same day the city declared a public health emergency. Charles Krupa/ASSOCIATED PRESS hide caption

itoggle caption Charles Krupa/ASSOCIATED PRESS

Shea Catlin, a nurse practitioner, doses out flu vaccine to give a shot at a CVS Minute Clinic in Arlington, Va., on Jan. 3. Barbara L. Salisbury/The Washington Times/Landov hide caption

itoggle caption Barbara L. Salisbury/The Washington Times/Landov