Based on a house-to-house investigation into the rumor spread phenomenon in the SARS Incident of Guangzhou during March 8-9, 2003, our main conclusion is that the circulation channel of rumors is chiefly personal of human spreads. The core of those rumors was true, whereas most of their surface details false. The synthetic reaction of both produced such a situation in which the public sought to prevent this unknown disease, but they failed to take proper measures due to a lack of sufficient and correct information. Meanwhile, the public was seized with a panic. Once the truthful part of rumors was verified through an official channel, it became a "message". Those false parts were denied, but they still existed or disappeared in the form of rumors. The rumor played a role of "anti-power". It compelled the "authoritative" official channel to make open responses. The rumor, developing to a certain extent, already completely became a destructive force so that normal social life could not go on. At last, the crisis took a turn for the better. The government and media began to change their attitudes, making a lot of open reports about the disease. Thus, the "SARS Incident" turned to a new stage in which governmental departments took active measures for prevention.