More than 400 schools around the United States have shut their doors, dismissing nearly a quarter of a million students as fears grew over the spread of a potentially deadly strain of flu.

“Schools are not only very good for education but also for the transmission of illness,” said Fran Phillips, deputy secretary for public health in the state of Maryland, where nine probable cases of the H1N1 virus have been detected, including several among school students.

Officials in Forth Worth, Texas, closed all 144 schools in their district and cancelled extra-curricular activities after a case of the H1N1 virus was confirmed.

In the Washington commuter suburb of Rockville, officials on Friday cited an “overabundance of caution” as they ordered the indefinite closure of a high school after a student took ill.

“This is not the first probable case of H1N1 flu among school students, but it is the first school closure,” Ulder Tillman, the health officer for Maryland’s Montgomery County, where Rockville High is located, told a news briefing. “Because this student has special needs and is unable to follow instructions to always cover his mouth when he coughs or sneezes, we took the decision to close the school.... Our aim is to stop or slow down transmission in the community,” she said.

The United States, the second hardest-hit nation, has confirmed 160 cases of H1N1 in 21 states.