Read previewBird flu is flying wild, and it has many infectious disease experts more worried now than ever. The H5N1 avian influenza virus has killed tens of millions of birds across the planet and more than 40,000 sea lions and seals. Most people seem to have very little chance, if any, of catching H5N1 avian influenza right now. Jim Vondruska/ReutersBut infectious disease experts are increasingly concerned that the H5N1 virus could make a sustained jump into humans and start spreading among us. This virus is a leading candidate for the next pandemic, and four developments in the past month have experts worried.

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