Klez topped the chart for the most common virus infection for the 15th consecutive month, security firm Sophos said yesterday.
And while Klez refuses to go away, a new entry called Datemake made the chart for the first time this month.
This type of malware is known as a dialler and is programmed to call a premium rate telephone line, typically with the intent of gaining access to adult material.
Chris Belthoff, senior product marketing manager at Sophos, said in a statement: "Businesses should apply strict computing guidelines to prevent getting stuck with a huge telephone bill, not to mention the potential embarrassment brought by these seedy programs."
In addition to the 475 previously undocumented viruses, worms and Trojans which Sophos detected in April, the company also warned that hoax emails, such as WTC Survivor, continue to clog up corporate networks.
"The WTC Survivor hoax continues to fool many computer users. A newspaper in the United Arab Emirates recently warned readers to be ready for imminent attack based on the hoax," said Belthoff.
"People who receive this hoax shouldn't believe everything they read. Instead of forwarding the warning to everyone they know, they should seek the advice of their network manager."
April 2003 virus chart:
1. W32/Klez-H (Klez variant) 12.7%
2. W32/Lovgate-E (Lovgate variant) 4.9%
3. W32/Bugbear-A (Bugbear worm) 4.3%
4. W32/Sobig-A (Sobig worm) 3.3%
5=. W32/ElKern-C (Elkern variant) 2.9%
5=. W32/Yaha-E (Yaha variant) 2.9%
5=. W32/Yaha-K (Yaha variant) 2.9%
8. JS/NoClose (NoClose worm) 2.1%
9. W32/Flcss (FunLove) 1.9%
10. Dial/Datemake-A (Datemake dialer) 1.6%
IBM and Technical University of Munich team demonstrate how Shor's algorithm, which can't be cracked by conventional computers, can be solved quickly with quantum computing
Hubble Space Telescope finds superflares from young red dwarfs could strip away planetary atmosphere
Younger stars are 100 to 1,000 times more energetic than when they're older
Two of the big four supermarkets will use the system to control sales of restricted products
PUBG news and updates: November's Update #23 to bring new Skorpion pistol and changes to blue zone visibility
Genuinely useful side-arm coming to PUBG in Update #23