All eight flu samples evaluated by the Alabama Department of Public Health have been found to be negative for swine flu, an agency press release reported early Wednesday afternoon. But, later Wednesday, the department reported two possible cases of swine flu in students at a Madison County elementary school.

The two students attend Heritage Elementary School in Madison County, according to the release, and there was no indication that the children had traveled recently to Mexico.

It was also reported that other children at the school were also showing symptoms of respiratory illness and that officials are recommending the school be closed for at least seven days.

This report comes the same day as the number of confirmed cases in the United States has increased and cruises out of Mobile to Mexico have been cancelled.

In an Associated Press report Wednesday, Carnival Cruise Lines said cruise ship passengers returning to the Mobile port will be observed for any signs of swine flu.

Carnival’s Holiday returns Thursday from a Gulf of Mexico cruise that stopped Monday in Progreso, Mexico, but did not make its scheduled stop in Cozumel.

In a statement, Carnival Cruise Lines said nobody on their ships has had swine flu. The company also said it will use alternate ports at least through May 4 because of the flu threat.

Federal Border Patrol agents will be watching passengers closely and looking for signs of sickness as passengers leave the vessel in Mobile.

According to the Department of Public Health, the incubation period from the moment of exposure to swine flu until symptoms develop is two to seven days.

“For example, a person who traveled to Mexico three weeks ago and who has not been ill should not be concerned about exposure from that location,” state health officer Dr. Donald Williamson said. “More than half of confirmed U.S. cases have no exposure from travel to Mexico.”

The swine flu virus is a respiratory infection caused by influenza type A viruses that typically cause outbreaks of influenza in pigs. People do not normally get swine flu, but human infections can occur. Human cases typically involve people who have had direct contact with pigs, but person-to-person transmission is suspected among recent cases.

“The best way to prevent spread is through following the basic rules of hygiene,” Williamson said.