The top 10 virus threats during May were all worm-related, according to a survey from security company Trend Micro.

During the month the company detected 1050 malicious codes, compared to 1700 the previous month. Malicious codes could include computer worms, viruses, trojans, and other malware.

Trend Micro in May also issued its first red alert for 2004 -- for the Sasser worm. Yellow alerts were also issued for worm_sasser.A and worm_wallon.A.

In a statement, Trend Micro emphasised the importance of keeping operating systems and applications up-to-date. 'The best way to ensure systems are protected against these malicious codes is to apply the security patches provided by Microsoft,' according to Trend Micro.

The company also found the majority of threats during May still related to the Netsky family of worms. It said this emphasised the 'ferocity of the 'war' between the Bagle and Netsky writers'.

'8 May saw the arrest of the suspected creator of Netsky (and reputedly Sasser) worms in Germany,' it stated. 'It is possible, however, that this one person was not solely responsible for all the variants of Netsky -- particularly as new variants of the Netsky worm appeared just days after the suspect's arrest.'