The H1N1 influenza outbreak crossed the 150 mark in the U.S. on Friday, as new cases were reported in Delaware, Texas, Illinois, Kentucky, South Carolina and Tennessee, bringing the total to 162 by evening.
The Monday death of a 22-month-old Mexican toddler in Houston remains the only confirmed fatality outside Mexico from the disease, unofficially known as swine flu.
About 430 schools have been closed nationwide, 300 of them in Texas and 62 in Alabama. That represents less than half a percent of the nation's 100,000 schools, according to the Department of Education.
Those two states have also canceled high school athletic events for the time being. Some colleges have called off graduation ceremonies, and the UC Regents changed a meeting scheduled next week in San Diego, saying they would meet instead by phone and at other campuses.
Harvard Medical School in Boston suspended classes until at least Wednesday after nine students in the dental school were identified as probable cases. One of the students contracted it from a friend who traveled to Mexico and apparently passed it on to classmates, officials said.
Dr. Anne Schuchat of the Centers for Disease Control and Prevention said a preliminary study indicated that in U.S. households with an infected victim, about a quarter of other household members also contracted the virus -- a relatively modest proportion. In some pandemics, the rate has been as high as 35%.
Dr. Nancy Cox of CDC also said Friday that the new virus apparently lacks some of the genes that made the strain in the 1918 Spanish flu pandemic so deadly.
"We do not see the markers for virulence that were seen in the 1918 virus," she said.
However, she added, "there is still a great deal that we do not understand" about the H1N1 virus and its counterpart 91 years ago.
Mexican Health Secretary Jose Angel Cordova said Friday that the confirmed death toll from the H1N1 flu outbreak in his country was 16 and that the total number of cases nationwide had risen to 397. The country stopped reporting suspected infections when the number reached 2,500.
In a new report, however, the CDC said that it had confirmed H1N1 flu in 84 Mexican patients who died.
The figures have risen as samples from past flu patients are tested for the newly discovered virus.
Cordova also said public hospitals that cover about half the country had admitted 46 patients with severe flu symptoms Thursday, down from 212 on April 20.
