NEW YORK - Don’t let your antivirus software and your firewall lull you into a false sense of security.

A new report on the state of malware, the insidious software designed to ruin computers and steal data, has identified two disturbing trends: new social engineering tricks that exploit current events, and the fast proliferation, via spamming, of viruses that would otherwise spread slowly.

The report, by Sunnyvale, Calif.-based Fortinet, a provider of security software, listed the most frequently detected virus threats from June 1 through 24. While mass-mailer worms accounted for 68% of total global activity, Fortinet expressed alarm at the rise in Trojans and backdoor viruses, which made up 5% of the global virus activity in June.

“It marks a more than 100% increase on last month’s Trojan activity due to a new trend in virus propagation strategies,” says Guillaume Lovet, team leader for the Threat Response Team at Fortinet.

Lovet says the inclusion of Trojans, a type of virus that doesn’t replicate or spread on their own, in the top 20 threats for the month was surprising. By spamming Trojans, mass-mailers are spreading the program faster and compromising more computers than in the past.

In May, Fortinet noted that e-mails about FIFA World Cup soccer tickets had virus-laden attachments. The trend continued in June, with mass-mailers and scamsters using current news events to trick users into clicking on an attachment. One spam e-mail announced the capture of Osama Bin Laden and promised pictures of the capture in the attachment. By clicking on the attachment, the users unwittingly installed a Trojan, a program that opens a backdoor to a user’s machine allowing more viruses to creep in.

The problem could become pronounced as more Internet users sign up for automated news alerts and RSS feeds, since they wouldn’t think anything strange about receiving this seemingly permissible information in their e-mail inbox.

Another spam announced a suicide attempt by Michael Jackson and directed curious readers to a link to read the article, but the link actually pointed to a Web page loaded with browser exploits to install software that seriously compromises the security of a computer.

Variants of the worm Mytob were detected most frequently in June, accounting for about 40% of total virus activity. Unlike a Trojan, worms self-replicate by attaching themselves to outgoing e-mail messages. Fortinet called June a “Mytob Month,” noting that over 150 distinct variants of the worm were detected, compared to only 30 in May.

The report called the Mytob infections a “literal storm” to gain control of other computers on the Internet. Each computer infected by a Mytob variant becomes a “zombie,” that is, ready to be sold to someone who wants to use the machine for shady activities, including mass-mailing spam, phishing messages or launching other viruses.

Lovet says spyware authors buy or rent already-compromised machines to “plant” their own malware products.

Fortinet predicted an increased number of focused attacks. At the start of June, a Trojan was discovered that stole thousands of confidential documents and data through a backdoor in a company in Israel. It had no particularly revolutionary features, other than it had been created to target that single company. Many antivirus products would have caught the Trojan, but precautions failed in this instance because the program was written specifically to attack the particular company.