NEW YORK (CNNMoney.com) - What do Trojan Horses, the Kama Sutra and a worm have in common?

All are malicious software that attack computers and left businesses vulnerable in February, according to Sophos, a Lynnfield, Mass.-based company that makes anti-virus and anti-spam software for corporate customers. The company was the first to discover the OSX/Leap-A worm, the first known worm designed to attack Macs.

New threats from "malware," or programs designed to attack computers, rose 48 percent from 2004 and 2005, according to Sophos, which expects another increase in 2006. These attacks are often financially motivated -- the makers of malware hope to build so-called "zombie networks" of e-mail addresses that can be rented out to people who want to send large amounts of spam.

Malware threats come in a variety of forms, according to Sophos. One of the most widespread in February was the Clagger-G virus, a so-called "Trojan Horse," or virus that frequently appears as a joke program or other type of software. The virus debuted on Sophos' monthly top 10 list of viruses in February at No. 8.

"What we saw this past month was a significant increase in the number of Trojans," said Ron O'Brien, a senior security analyst at Sophos. "That is a program that embeds itself in the computer and does something when you least expect it."

Trojan Horses do not replicate or copy themselves and must be sent from one user to another via an e-mail attachment or another program. They're meant to intentionally disrupt and damage a computer's activity. While Trojans can't spread on their own, a user does not have to open an e-mail attachment to activate this type of virus.

"Backdoor Trojans" allow a user's computer to be operated by someone else unknown to them. This explains how computer users can receive e-mails containing viruses from people they personally know.

"If you get a message from someone you think you know, generally it's because that person's computer has been compromised," said O'Brien.

O'Brien noted that Clagger-G's presence in the top 10 was surprising, as it had to reach "an exorbitant number" of e-mail addresses worldwide to crack the list.

The second most widespread virus in February -- Nyxem D, aka the Kama Sutra worm -- debuted on Jan. 18. The e-mail worm uses pornographic disguises in an attempt to spread and disable security software.

Worms do not need host programs; these bugs can replicate themselves and are frequently spread via e-mail or the Internet. However, they cannot be activated unless a user opens an attachment containing the worm. Simply ignoring such e-mails seems obvious enough, but O'Brien said human nature often gets the better of users' common sense.

"Good things do not come in e-mails marked 'kama sutra'," O'Brien noted dryly. "Unless you know who the e-mail is from, under no circumstance should you open an attachment, any more so than you'd open an e-mail from a suspicious source. It requires some restraint on the part of the recipient."

Roughly one in 44 e-mails contains a virus, according to Sophos's research. When virus activity spikes, that number can increase to as many as one in 12.

In addition to ignoring e-mails from unknown sources and not opening attachments from unrecognized senders, O'Brien recommends abstaining from forwarding jokes or other content and also not opening the messages.

And be sure to regularly download security updates for your operating system and software programs.

Finally, be aware that clicking on an "unsubscribe" option in an unsolicited e-mail is often a means of capturing your e-mail address.

"If you click on unsubscribe, you have identified to the person who sent you that spam that you are a real person," said O'Brien. "The price of your e-mail address just went up exponentially."

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