Workers with the aid group Doctors Without Borders prepare a new Ebola treatment center near Monrovia, Liberia, on Sunday. The facility has 120 beds, making it the largest Ebola isolation clinic in history. John Moore/Getty Images hide caption

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Marqui Ducarme is aided by his wife after catching chikungunya at his home in Port-au-Prince, May 23. The virus swept through Haiti this spring, infecting more than 40,000 people. Marie Arago/Reuters/Landov hide caption

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A man lies in a newly opened Ebola isolation center in a closed school in Monrovia, Liberia, on Thursday. The official death toll of 1,000 people in four countries is likely below the actual number, the World Health Organization says. John Moore/Getty Images hide caption

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Better than Egyptian cotton: This electrically spun fabric contains anti-HIV drugs and dissolves rapidly when it gets wet. Courtesy of University of Washington hide caption

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An outbreak of bird flu in India in 2008 prompted authorities to temporarily ban the sale of poultry. Diptendu Dutta/AFP/Getty Images hide caption

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The usual suspect: Bats harbor dozens of deadly viruses, such as rabies and influenza. Several studies suggest that bats may also carry Ebola. Tyler Hicks/Getty Images hide caption

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Yonta, 6, rests with her sister Montra, 3, and brother Leakhena, 4 months, under a mosquito net in the Pailin province of Cambodia — an epicenter of drug-resistant malaria. Paula Bronstein/Getty Images hide caption

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When Sule Koroma's sister Saudatu died of Ebola, his house in Freetown was put under a 21-day quarantine. The government sent two policeman to guard the house. Tommy Trenchard for NPR hide caption

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Dr. Kent Brantly, of Fort Worth, Texas, was treating Ebola patients in Monrovia, Liberia, when he himself became infected with the virus. Courtesy of Samaritan's Purse hide caption

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A health worker vaccinates a child during a polio campaign in Bannu, Pakistan, June 25. The Taliban threaten to kill vaccinators and parents who immunize their kids. A Majeed/AFP/Getty Images hide caption

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Red Cross volunteers prepare to bury the body of an Ebola victim in Pendembu, Sierra Leone, early this month. Tommy Trenchard for NPR hide caption

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