The risk of developing severe acute respiratory syndrome (SARS) after exposure was conventionally determined by the prospective follow-up for symptomatic disease or the retrospective seroprevalence study of the exposed population. The average number of secondary cases resulting from a single case of SARS ranged from two to four.1Transmission mostly resulted from contacts with patients with overt disease rather than from asymptomatic or mildly symptomatic patients. Seroprevalence appeared to be low (0%, 0.43%, and 1.2%) for healthy individuals, and about 1% for health-care workers, approximately 1% for asymptomatic family contacts under quarantine, and 0.19% for asymptomatic contacts overall.2–7 Systematic use of reverse transcriptase polymerase chain reaction (RT-PCR) in the early identification of patients with higher risk for developing SARS has not been reported. In this issue of CHEST (page 95), Ho et al8 report on the nasopharyngeal shedding of SARS-coronavirus (CoV) RNA from 27 of 217 frontline health-care workers (12.4%) after encountering SARS patients for 1 week. Twenty five of those health-care workers were characterized by low mean (± SD) viral loads (312 ± 204 to 386 ± 203 copies per milliliter), a lack of or paucity of symptoms, and the absence of seroconversion during follow-up. This is in contrast to the two subsequently symptomatic health-care workers with significantly higher mean viral loads (16,900 ± 7,920 copies per milliliter) and subsequent seroconversion. The authors excluded contamination with PCR amplicon carryover by using 13 nonfrontline health-care workers as negative control subjects in addition to the usual PCR-negative control subjects. Since the word colonization is used to describe the establishment of a microbial agent in the host without inducing a specific immune response or invasion, as manifested by disease or distant dissemination, the authors concluded that SARS-CoV can “colonize” a significant proportion of exposed individuals, with disease manifestation occurring in only 2 of 27 initially colonized individuals (7.4%).