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Researchers are scrambling to ascertain how readily a virus that has infected hundreds of people — and killed at least four — in Asia spreads between people. The revelation on Monday that health-care workers have been infected, considered a litmus test for a pathogen’s ability to spread between people, suggests that the pathogen is more contagious than previously thought, they say. The World Health Organization has called a meeting to decide whether to declare the worsening outbreak a public-health emergency.