http://www.washingtonpost.com/world/asia_pacific/a-new-case-in-chinas-capital-adds-unknowns-to-spread-of-latest-bird-flu/2013/04/15/f6b9827e-a5d3-11e2-9e1c-bb0fb0c2edd9_story.htmlThe appearance of cases with mild or no symptoms in humans could make tracing even more difficult, but may also mean that many people infected do not get seriously ill and recover quickly, making the virus is less deadly than it appears.
（实际可能比这低，
因为那些病情轻的患者因为不知道得了禽流感而没有被统计进去）