The handful of new Sars cases in China cast a pall over the May Day celebrations. The government has reacted promptly and transparently to contain the spread of the disease. Nevertheless, at least one patient has died.
But how did the virus return? Experts are certain that the cases are all linked to what appears to be a laboratory accident at the Chinese Centre for Disease Control and Prevention (China CDC), the very organisation charged with preventing and controlling epidemics.
We do not know how the two students - a Ms Song and a Mr Yang - connected with the Institute of Virology at the centre contracted Sars. There was a 23-day gap between the time Ms Song fell ill and when Mr Yang developed symptoms.
Either both made mistakes, although their duties did not require them to handle the virus, or someone else was careless and infected them. Whatever the case, the China CDC cannot shrug off responsibility for failing to enforce strict rules and procedures.
Management and staff obviously have not learned from mistakes made last year in laboratories in Singapore and Taiwan, which both led to the infection of a researcher. The China CDC did not strengthen the management and security of research labs, or block potential leakage channels. Officials did not consider early enough the possibility that the sick students had been infected with Sars, even when they had high fevers.
Compounding those errors, the hospitals treating the patients did not make the correct diagnosis in time. The reason for the delay: neither the victims nor their employers immediately alerted doctors to the possibility that they had been in contact with the Sars pathogen.
Ms Song, who travelled more than 1,000km by train between Beijing and Anhui province after she was infected, passed the virus to a nurse who treated her, her mother - who has since died - and perhaps others. Nearly 1,000 people who came into contact with Ms Song and those she infected have had to be quarantined. For all this, the China CDC must take the blame.