“At present, the cases have been sporadic in nature, and no evidence of human-to-human transmission has been discovered,” Mr. Liang said, but he cautioned that “currently, due to the limited understanding of the origin and dissemination of this disease, there are still uncertainties remaining as regards the aspects of epidemic prevention and control work.”

Chinese officials are still investigating two families in which multiple cases of H7N9 are suspected, according to the W.H.O. One of the cases involves an 87-year-old man who died on March 4 and was later found to have had the disease. Two of his sons developed severe pneumonia about the same time, and one died on Feb. 28. But while severe pneumonia is rare, H7N9 has not been confirmed in either of the sons. And even if it had, the sons, as well as their father, could have been infected through animals instead of humans.

The Chinese authorities have not released details of the other family under investigation.

Mr. Liang said the Chinese government would be open about the outbreak’s progression, and the W.H.O. said it was in discussions about whether to send international experts to work with China in halting the spread of H7N9.

Xinhua, the state-run news agency, said one of the 24 people who were infected, a 4-year-old boy in Shanghai, appeared to have recovered, although he remained in a hospital for observation. While the boy’s improvement is a sign that the disease is not necessarily fatal, it also raises the question of whether more people are coming down with mild or even asymptomatic cases and recovering without becoming sick enough to visit a hospital or clinic for testing.

Because the virus can continue to mutate in people who are infected, scientists are eager to know whether H7N9 manifests itself only in cases that are serious enough to lead victims to seek medical attention. The W.H.O. lists 3 of the 24 cases as mild, although those patients were still in a hospital.