A virulent computer bug has infected systems at more than 150 of New York City’s 1,350 public schools, preventing students, teachers, and administrators from using the Internet and e-mail.

Some principals said they first detected the virus a month ago. Others said they noticed something amiss in the last couple of weeks.

The Department of Education’s division of instructional and information technology realized there was a problem when it detected abnormal network activity at a number of schools. The division took action Monday, sending a message out to regional administrators, which was forwarded to individual schools. The division also started deploying teams of technicians to schools to try to contain the problem.

“We’ve taken immediate steps to contain the viruses at the approximately 150 infected schools,” an official of the education department said. “We have also taken measures to minimize the disruption to the instructional environment while we execute the clean and eradicate phases.”

The counteroffensive is targeting viruses called Blaster Worm, Welchia Worm, and Sobig. All three affect Windows systems, not Macs.

An education official said all the viruses were propagated through the Internet. “They infect machines that are not protected with updated antivirus software,” the official said. “Once infected, an outside hacker can utilize that machine to attack other machines on the network.”

That means infected computers have been sending out mass e-mails, clogging the network with excess traffic, and infecting recipient computers.

A spokeswoman for the education department, Michele McManus, said the city government was chipping in to combat what she called the “particularly virulent virus.”

“Security experts from the city are assisting us in determining the amount of time and resources required to complete the cleanup activity,” she said.

The viruses have infiltrated computer systems in all five boroughs, across grade levels, with some high schools, middle schools, and elementary schools all affected.

School employees who would discuss the problem asked not to be identified.

One official of an elementary school said her school’s offices and computer labs have been without Internet access for a week. During that time, the school didn’t receive any of the messages it was supposed to receive from the Department of Education and its regional leaders.

She said the school had been affected by computer viruses in the past, but they had never infected so many computers or persisted so long. By yesterday afternoon, most of her school’s computers were up and running, but some were still slow or not working properly.

An official of a high school said the viruses were “slowing everything down” and keeping people from sending and receiving e-mail messages.

An official at another high school said it’s important for simple systems to work at schools. He said it seems as though when the education department decided to turn the old paper-based system into a computerized system, it did not install the right firewalls and protections to guard against viruses.

“They were supposed to make this an efficient, paperless environment,” the official said.”…You really have just the opposite. Sometimes simpler is better.”

The department acknowledged yesterday that only machines “not protected with updated antivirus software” can be infected with these viruses. “We just thought it was our system that was bugged,” the high school official said, adding that it was only after a few days that they found out other high schools shared the problem.

He said his school is having trouble with its Comprehensive Attendance, Administration, & Security System. That’s the system that collects student attendance data in the mornings, when children scan their photo identification cards upon entering the school. He said usually the system alerts officials when a truant or a suspended student tries to scan into the system. That isn’t happening now.