The Rev. Herman Browne voluntarily quarantined himself for 21 days after his wife's friend tested positive for Ebola. On Sunday, he returned to his church, Trinity Cathedral, to preach to his congregation about Ebola prevention. Jon Hamilton/NPR hide caption

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Avian influenza, or bird flu, causes an infectious and contagious respiratory disease. In the lab, several scientists have made the H5N1 strain more contagious, a controversial line of research. James Cavallini/ScienceSource hide caption

itoggle caption James Cavallini/ScienceSource

An ambulance carrying Amber Vinson, the second health care worker to be diagnosed with Ebola in Texas, arrives at Emory University Hospital in Atlanta on Wednesday. David Tulis/AP hide caption

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Icon Genetics' Dr. Frank Thieme selects samples of Nicotiana benthamiana, a relative of tobacco, growing in a company greenhouse in Halle, Germany. The company uses the plants to produce antibodies that could be helpful for increasing supplies of ZMapp. Sean Gallup/Getty Images hide caption

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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

Children's Mercy Hospital in Kansas City, Mo., was the first to report a surge of children with serious respiratory illness in August. Andy Pollard/Children's Mercy Kansas City hide caption

itoggle caption Andy Pollard/Children's Mercy Kansas City

Sofia Jarvis, seen here with her family at a press conference in February, is one of several dozen children in California who have been diagnosed with a rare paralytic syndrome. It has left her left arm paralyzed. Martha Mendoza/AP hide caption

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Traffic moves past Texas Health Presbyterian Hospital in Dallas, where a patient showed up with symptoms that were later confirmed to be Ebola. Mike Stone/Getty Images hide caption

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A patient at the Texas Health Presbyterian Hospital in Dallas has a confirmed case of Ebola, the Centers for Disease Control and Prevention says. He is being treated and kept in strict isolation. LM Otero/AP hide caption

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Biohazard suits used to handle dangerous microbes hang in a laboratory at the U.S. Army Medical Research Institute of Infectious Diseases in Fort Detrick, Md. Patrick Semansky/AP hide caption

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Low water levels, like at this reservoir near Gustine, Calif., bring birds and mosquitoes together and help transmit West Nile virus to humans. Rich Pedroncelli/AP hide caption

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Chlorine can stop the Ebola virus. So medical workers disinfect their hands often at the Doctors Without Borders treatment center in Kailahun, Sierra Leone. Carl De Souza/AFP/Getty Images hide caption

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San Francisco Supervisor Scott Wiener (left) says he started taking a drug to prevent HIV infection earlier this year. Lisa Aliferis/KQED hide caption

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Workers unload medical supplies to fight the Ebola epidemic from a USAID cargo flight in Harbel, Liberia, in August. John Moore/Getty Images hide caption

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Medical workers at the John F. Kennedy Medical Center in Monrovia, Liberia, put on their protective suits before going to the high-risk area of the hospital, where Ebola patients are being treated, Sept. 3. Dominique Faget/AFP/Getty Images hide caption

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