A Missouri resident who shared a home with a patient hospitalized with bird flu in August was also infected with the virus, federal officials reported on Thursday.

But symptomatic health care workers who cared for the hospitalized patient were not infected, testing showed. The news eased worries among researchers that the virus, H5N1, had gained the ability to spread more efficiently among people.

Still, the number of human cases is rising in the United States. California said this week that it had confirmed 15 human cases of bird flu. Washington State has reported two poultry workers who are infected and five others presumed to be positive.

There are 31 confirmed cases in the country, but experts have said the figure is likely to be an undercount. “Additional cases may be found as investigations continue,” Dr. Nirav Shah, the principal deputy director of the Centers for Disease Control and Prevention, said at a news briefing on Thursday.

“The identification of these additional cases of H5 in people with exposures to infected animals does not change C.D.C. risk assessment for the general public, which continues to be low,” he said.

The poultry workers in Washington State were infected with a version of the virus that is distinct from the one circulating in dairy cattle, he added.