By Mark Ward BBC News Online technology correspondent

Viruses are getting sneakier

So says Steve Chang, head of anti-virus firm Trend Micro, who predicts that the business of selling security software is due for a shake-up.

With viruses getting harder to defend against, anti-virus firms will have to start offering security services not just software.

He says increasing numbers of users and customers want someone else to worry about viruses, so they can get on with their job or life.

Nasty programs

Mr Chang, a pioneer in the computer security world, said anti-virus companies once compared themselves to pharmaceutical firms that developed an antidote when a new virus appeared.

But the growing ubiquity of the net and the appearance of viruses such as Nimda, Slammer and Bugbear has, Mr Chang believes, changed this cosy situation.

The ability of a company to avoid infection now depends on the security measures of all the other businesses, organisations and individuals it has net links with, as much as it does on that firm's own diligence.

A virus outbreak can cause huge disruption

For instance, workers who neglect security when working at home can unwittingly contract a virus and unleash it inside a company network when they get back to the office.

Other viruses flood net links with traffic, causing delays and frustration for anyone else using the same pipes for their own web access.

Also, says Mr Chang, there are some viruses that are almost impossible to defend against because they roll so many attack techniques together in one malicious package.

This will force a change in the way that anti-virus firms do business, he says.

"If you cannot solve the virus problem you need to minimise the downtime," he told BBC News Online.

Big changes

With anti-virus software becoming a commodity, firms that make it will turn into security service providers, he says.

Companies increasingly want a service they subscribe to that protects them from attack and speeds up the recovery after an outbreak.

Avril Lavigne: Pop star and now a virus

Customers have never wanted anti-virus software for its own sake, says Mr Chang, instead they wanted it to ensure they could get on with what they really want to do.

"You buy a drill because you want a hole," says Mr Chang, "not because you want a drill."

As a result Trend Micro is now changing to start offering an anti-virus service rather than just software and regular updates.

Trend has signed deals with net service firms who then give users basic protection for a monthly fee.

Many of Trend's business customers are signing up for similar services to let someone else worry about the viruses.

"Customers do not want a security product," says Mr Chang, "They just want to improve their productivity without downtime."