A child wears a mask near a closed section of a poultry market in Shanghai, where health workers detected the new bird flu, H7N9. Eugene Hoshiko/AP hide caption

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A cockerel walks on a bridge in a residential area of Beijing. The Chinese are beginning to destroy thousands of birds in an effort to stamp out the presumed source of H7N9 infection. Wang Zhao/AFP/Getty Images hide caption

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A child is immunized against polio at the health clinic in a farming village in northern Nigeria. The procedure involves pinching two drops of the vaccine into the child's mouth. For full protection, the child needs three doses, spaced out over time. David Gilkey/NPR hide caption

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About a third of the world's population is thought to be infected with Mycobacterium tuberculosis, but only a small fraction of people get the disease. NIAID_Flickr hide caption

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Although tuberculosis is declining around the world, drug-resistant strains of Mycobacterium tuberculosis are on the rise. NIAID/Flickr.com hide caption

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Klebsiella pneumoniae, seen here with an electron microscope, are the most common superbugs causing highly drug-resistant infections in hospitals. Kwangshin Kim/Science Source hide caption

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A new coronavirus looks a lot like its cousin SARS under the microscope, but it appears they're quite different when it comes to contagiousness. NIAID/RML hide caption

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Health officials around the world are on constant lookout for the deadly bird flu. Here a worker collects chickens on a farm in Kathamndu, Nepal, where the virus was suspected of infecting poultry last October. Prakas Mathema/AFP/Getty Images hide caption

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Coronaviruses have a characteristic crown of tentacles when viewed under the electron microscope. BSIP/UIG via Getty Images hide caption

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Virologists discovered the new coronavirus after it killed a Saudi Arabian man last summer. Elizabeth R. Fischer/Rocky Mountain Labs/NIAID/NIH hide caption

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An artist on Rio de Janeiro's Copacabana beach puts the final touches on a sand sculpture of the kissing bug, which spreads Chagas' disease. The sculpture was part of an event in 2009 commemorating the 100th anniversary of the discovery of the disease. Vanderlei Almeida/AFP/Getty Images hide caption

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Health workers in Nepal culled chickens and destroyed eggs following an outbreak of bird flu in Kathmandu in October 2012. Prakash Mathema/AFP/Getty Images hide caption

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