The number of probable cases of the H1N1 flu virus, better known as swine flu, has quadrupled statewide in the last two days, with the latest figures showing at least 16 cases in Chicago.

City officials continue to be cautious, hammering a message of calm and prevention, urging people to wash their hands, cover their coughs and sneezes and teach those habits to others.

Chicago Public Schools announced yesterday that a North Side school, where a student was identified as having a probable case of the virus, would remain closed until further notice.

CPS CEO Ron Huberman says the administration was monitoring attendance rates to identify high instances of absences. The district is easing attendance policies in light of the swine flu threat.

While city officials have expressed strong concern, directing people to numerous Web sites for information, the city's Department of Public Health Web site had not been updated since Tuesday.

Meanwhile, city labor organizations issued a joint statement urging people to attend today's May Day immigration march in downtown Chicago despite swine flu concerns. 

"We should not politicize health issues. There is no reason to believe that a group of people at a political rally are any more likely to spread disease than a group of people cheering at a ball game or taking public transportation to work," said a statement released by the Chicago Federation of Labor, Service Employees International Union Local 1 and other unions.

A Chicago Blackhawks spokesman said the team, which is playing the Vancouver Canucks in the National Hockey League playoffs, continues to monitor the situation and is working with the NHL.

Mayor Richard M. Daley yesterday scoffed at remarks made by Vice President Joe Biden, who said people should avoid public transportation. Biden's office later clarified his remarks, saying Americans should take measures to stay healthy.

The numbers of probable cases have skyrocketed.

On Wednesday, IDPH director Damon Arnold said there were five cases in Chicago, including a 12-year-old girl who was hospitalized and a 20-year-old Loyola University student.

There are now 16 cases in Chicago. IDPH spokeswoman Kelly Jakubek could not say specifically where in the city the cases had appeared.

The latest figures, as of 6 p.m. yesterday, include 11 probable cases in Cook County, two in DuPage County, three in Will County, and one in Lake and McHenry counties.

Three young girls in Joliet were among the newest cases. On Wednesday, the first American fatality was confirmed, when a 23-month-old Texas infant died. More than 100 probable cases have been identified in the United States. Yesterday, Mexican officials declared the outbreak was leveling off.