The H7N9 virus , at least 571 of several strains of bird flu known to be able to infect humans , has persisted , diversified and spread in chickens across China . This has fulled a resurgence of infections in people , posing a wider threat . Unless control measures are put in place , the virus will spread , they warn .
The H7N9 virus , one of several strains of bird flu known to be able to infect humans , has persisted , diversified and spread in chickens across China . This has fulled a resurgence of infections in people , posing a wider threat . Unless control measures are put in place , the virus will spread , they warn .