Spread of Bird Flu Worries China Ahead of Peak Travel Time HONG KONG - China is disclosing a steadily growing number of cases of H7N9 bird flu, including four more cases announced on Friday, reviving concerns among health experts that the disease may be spreading and could pose a further threat as the world's largest annual human migration begins ahead of Chinese New Year. Mainland China has confirmed 14 cases this week alone, including the four announced on Friday, and seven on Thursday. Human cases of the H7N9 avian influenza virus began to emerge in late March near Shanghai, infecting 131 people, including 26 who died, by early May. The virus then seemed to fade away, as influenza viruses often do over the summer. The virus then re-emerged much earlier in the season last October with a trickle of cases, and that trickle has now accelerated in January. The accelerated tempo of cases comes just before people across China begin their traditional trips to family reunions for celebrations of the Lunar New Year, which falls on Jan. 31 this year. The official Lunar New Year travel season in China began Thursday, with the government estimating that 3.62 billion trips would be taken in the next 40 days by road, train, airplane and other modes of transportation. Health officials in protective suits transported sacks of poultry as part of preventive measures against bird flu at a poultry market in Zhuji, China, in January. Many of those trips are to hometowns in rural areas. A large majority of today's Chinese grew up in the countryside, even though the country as a whole became more than 50 percent urban in 2011 because of heavy migration to factories, construction sites and universities in cities. Contact with poultry, common in rural areas, is still the main route of infection for the virus. Heavy travel in densely packed vehicles offers the virus more chances to pass from person to person, and possibly evolve into new forms that may be more readily transmissible. Health experts are watching closely for two warning signs of greater human-to-human transmission that have not yet occurred on a large scale. One warning sign would be a spate of cases among people who have had no apparent contact with poultry or environments contaminated by the feces, uncooked blood or other fluids of poultry. The other warning sign would be a series of cases in which several members of the same family fall ill in quick succession and appear to have transmitted the disease to one another. Helen Yu, a spokeswoman for the Beijing office of the World Health Organization, wrote in an email that the proportion of cases among people who had no contact with poultry had stayed low since the disease emerged nearly a year ago and showed no sign of increasing this winter. Similarly, there has been only one family cluster of cases this winter, compared with four clusters last spring. "It is possible that limited human-to-human transmission may occur, but there is no evidence of sustained or widespread human-to-human transmission," she wrote. We continue to expect sporadic human cases. Extensive testing for bird flu may also result in more cases being detected even if the actual rate of infections is not increasing as rapidly as the data on confirmed infections might suggest. Yet the H7N9 virus remains a particular concern for two reasons. It has a series of genetic mutations that have been associated in other viruses with greater adaptation to human-to-human transmission. And the H7N9 virus has proved itself to be "much, much better than other avian influenza viruses" at growing in human lung tissue samples in a laboratory at Hong Kong University, said Dr. Malik Peiris, a prominent avian influenza researcher at the university. The laboratory uses lung tissue that was removed from people during lung cancer surgery or other procedures. The tissue would normally be discarded after such surgeries but is sent to the laboratory for tests instead, Dr. Peiris said. The Center for Infectious Disease Research and Policy at the University of Minnesota noted in a short report on Thursday that the pace of new cases in China in recent days has already matched the busiest pace reached last spring. According to the W.H.O., 183 cases have been reported from mainland China since last March. Hong Kong has also reported three cases, and Taiwan has reported two, all of them involving people apparently infected in mainland China. The W.H.O. is not currently recommending any restrictions on travel to China. But the Geneva-based organization is suggesting that visitors to China avoid live bird markets or if they must visit them, that they avoid live animals and surfaces in contact with live animals or with the blood or feces of poultry. The organization is also recommending that poultry be cooked thoroughly, that poultry-cooking implements be cleaned thoroughly, and that visitors wash their hands regularly and cover their mouths and noses when coughing or sneezing. In a sign that governments around the region are starting to take precautions as well, Hong Kong announced late Friday that it would begin conducting blood tests on Jan. 24 on local and imported poultry to determine if they have the virus. Any birds with confirmed infections will be killed, as will any birds that have been kept with the infected birds, said Dr. Ko Wing-man, the secretary for food and health.