CHICAGO — Flu viruses must be able to pick a very specific type of lock before entering human respiratory cells, U.S. researchers said on Sunday, offering a new understanding of how flu viruses work. The discovery may help scientists better monitor changes in the H5N1 bird flu virus that could trigger a deadly pandemic in humans. And it may lead to better ways to fight it, they said. The scientists found that a flu virus must be able to attach itself to an umbrella-shaped receptor coating human respiratory cells before it can infect cells in the upper airways. ‘What the lock needs is the right key. It opens the door,’ said Ram Sasisekharan, a professor of biological engineering and health sciences at Massachusetts Institute of Technology in Cambridge. The H5N1 avian flu virus now almost exclusively infects birds. But it can occasionally pass to a person. Experts have feared that the bird flu virus would evolve slightly into a form that people can easily catch and pass to one another, triggering an epidemic. ‘We now know what to look for,’ said Sasisekharan, whose study appears in the journal Nature Biotechnology. Before a flu […]