pital system to control hospital cross-infection and infec- tion of visitors. Visits to mainland China were associated with SARS transmission, even after adjusting for other variables. Cross-border transmission played a role in the epidemic; although the absolute percentage is not high among the 1,192 case-patients (3.6% or 43/1,192), it is substantially larger among the undefined source group (12.4%). With a case-control design, we could not establish whether this 14.4% was associated with an inflated risk. Cross-border communication and prevention, such as those set in place (temperature screening and health declaration), need to be enforced strictly and consistently. Almost 70% of the 43 participants who visited mainland China had fever onset on or before April 1 (i.e., the early phase of the epidemic) (5). None of them had onset after May 3, which is under- standable as visiting mainland China was perceived as a high risk by the general public in the late phase of the epi- demic (5). The variables related to social contacts (with medical personnel or hospital visitors, with persons with influenza- like symptoms, and with persons living in a housing estate with a reported SARS patient) were not significant. These findings should be interpreted with caution. On one hand, these case-patients should not be stigmatized. On the other, the results may have been confounded because all SARS cases contracted this way were excluded from the analysis. However, confirming that these variables could not account for transmission of the undefined source cases can be useful. Evidence does not indicate that frequent visits to crowded places were associated with a higher likelihood of community-acquired infection. This finding may remove panic that arose during the epidemic, and daily life need not change as much as it had. Hong Kong is a densely pop- ulated city, and it had a large number of SARS cases. The number of community-acquired cases in less populated cities should be much lower than that of Hong Kong. This finding should be interpreted with care as >90% of the general public wore face masks in public places, and >85% avoided visits to public places during the epidemic in Hong Kong (5). Although visiting the Amoy Gardens was a risk factor, Amoy Gardens might be the only place where such a large-scale SARS outbreak was attributable to con- tamination of the environment. We now have some empirical evidence to suggest that wearing a face-mask frequently in public places, frequent handwashing, and disinfecting one’s living quarter were effective public health measures to reduce the risk for transmission (adjusted OR 0.58 to 0.36). The effectiveness of mask use was controversial (6). In another study, the prevalence of these three public health preventive public health measures increased significantly from March 21, SARS in Hong Kong 2003, to April 1, 2003, (i.e., wearing masks 11.5%–84.3%; frequent hand washing 61.5%–95.1%; home disinfection 36.4%–80%) (5). These practices played an essential role in limiting the spread of the virus in the community in Hong Kong. That disinfecting the living quarter is a strong protec- tive factor has a particular relevance. The reason behind the significance is not completely clear. During the epi- demic, the Hong Kong government released frequent announcements of public interest to promote home disin- fection using 1:99 bleach water solutions. Most respon- dents who disinfected their living quarters were probably following the government’s suggestion. Keeping in mind that probable secondary cases had already been removed from the analysis, such protective effect is not referring to the effects that disinfecting the quarter reduced the chance of secondary infection. Environmental contamination (sus- pected to be related to the sewage system) was reported in the Amoy Gardens, and similar environmental contamina- tion probably did not occur in other places. Such contami- nation-related infections might be on a small scale and not been noticed. In such circumstances, home disinfection might reduce the risk for transmission. The finding sug- gests that, in addition to the droplet theory, the fomites the- ory could not be dismissed. Our study has a few limitations as well as strengths. First, approximately 72% of all SARS case-patients were included in the study (excluding patients whose contact numbers were incorrect or not available; approximately 78% of those with a valid contact telephone number were included, and the refusal rate was about 10%). The sample size was reasonably large. Second, data were collected ret- rospectively. Most of the data were, however, collected from the participants within 1 month after onset of fever. Since contracting the disease is a major life event for the patient and family, they should be able to recall whether such factual and benchmark behaviors had been practiced. The study also has strength of matching for age, sex, and reference time of the behaviors in question, so that both the case and control in a pair were referring to rele- vant behaviors that occurred within the same 10-day peri- od before the date of onset of fever of the patient. Third, some questions, such as those about disinfection of house- holds or visiting crowed places were nonspecific (the ques- tions asked were “Whether your living quarter had been disinfected thoroughly” and “Whether you had visited crowded places”). Different participants might have defined the terms differently. Further, a number of patients were unable to answer the questions, and a household member who was “most familiar with the household situa- tion” was invited to serve as a proxy. The responses obtained from these informants were compared to those obtained from the patients themselves, and no statistical Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 10, No. 4, April 2004 591