The U.S. government declared a public health emergency Sunday to respond to the outbreak, which also has sickened people in New York, Kansas, California and Texas. Many of them had recently visited Mexico.

The child in Elyria displayed typical seasonal flu symptoms, including a sore throat and body aches, state health officials.

His family members did not have any flu-like symptoms, Barnes said Monday.

The boy, who returned from Mexico within the past two weeks, started feeling sick Wednesday, health officials said. He was treated for flu symptoms at a city health clinic, where an astute nurse practitioner began asking questions and realized he might have swine flu.

The student's contact with classmates last week didn't go beyond normal school activities, Higgins said.

Symptoms of the swine flu virus closely resemble those of the seasonal flu and include fever, weakness, coughing and lack of appetite.

The acting head of the Centers for Disease Control, Richard Besser, said Monday people can best protect themselves against the swine flu threat by taking precautions they were taught as children, like frequently washing their hands and covering their mouths when coughing.