MENIFEE -- While thousands of people in Riverside County scramble to find a flu shot for this year's nasty virus, Menifee school officials are dealing with another kind of virus.

Instead of affecting employee immune systems, this virus has affected about 1,000 computer work stations at schools and buildings in the Menifee Union School District, technology officials said this week.

Although only about 100 computers were actually infected, the rest of the machines had to be shut down to allow staff to figure out what was wrong.

Employees first noticed something might be wrong on a handful of computers on Nov. 15, said Steve Thornton, the district's technology director. Excessive popup ads when employees would access the Internet were the first signs that the district's security programs had been breached, he said.

Officials expect to squash the bug and have all of the district's computers back online by the end of next week, almost three weeks after the virus was discovered.

It's taking them so long to correct the problem because none of the major software companies have come up with a tool to combat this new variant virus, so district technology workers have been working on individual computers, cleaning out files and restoring their functions to a normal speed.

This is a slow process, and done only for computers that provide essential services, such as those in the business office, he said. It's not a solution for the whole network, but technology staff are trying to roll out a new fix they're developing in the lab that would seek out the unacceptable behaviors on network computers, and then paralyze those functions until a more sophisticated removal tool is developed.

Thornton said he believes someone opened an e-mail while using an outside mail server and the e-mail contained an innocuous-looking attachment that turned out to hold a virus.

He said such viruses are called trojans, because they look innocent, but are actually virulent. Menifee's virus also has elements of a worm, which will use information inside the host computer to spread itself.

Although employees can no longer access outside mail servers such as Hotmail or Yahoo on district computers, officials say the change is necessary to prevent a similar virus from infecting their systems.

New viruses and bugs are being developed all the time, and Menifee's virus filter software did not recognize the application, he said.

"There was nothing on the Internet to warn us, and none of the big guys have come up with a solution," he said.

Thornton said the district enabled employees to use servers such as Yahoo and Hotmail because staff will use it to keep in contact with parents, or other community members, along with using the district's e-mail.

"We choked that (service) off as a result of this," he said. "That was a big hole."

Dan Wood, assistant superintendent of business services, said his office's computers were down for about two days, and employees worked on anything that didn't involve computers in the meantime.

"We did whatever we could to buy time," he said. "In my case, I always have a lot of work that doesn't involve the computers."

Betti Cadmus, a reading coordinator and district spokeswoman, said she's been preparing reports by hand that track volunteer hours for the reading program. It's a long and cumbersome process and she's still working on it, although it's past due.

Access to Web-based grading software was initially shut off, as well, but teachers now can access it from home or from computers that have been manually cleaned at their school if their classroom computer is still down, Thornton said.

Contact staff writer Kelly Brusch at (951) 676-4315, Ext. 2626, or kbrusch@californian.com.