Computers in Japan are much less likely than those in other nations to harbour spam-spreading viruses and Trojans, according to vnunet.com's analysis of recent data on unsolicited email sources.

However, the reasons for this remain unclear. Recent data shows that the amount of spam coming from a country appears closely related to the number of broadband internet connections in that country. 

For example, the US has 21 per cent of the world's broadband lines, and accounts for 24 per cent of the spam relayed worldwide, while China is home to just under a fifth of broadband connections, and emits about a fifth of all of spam.

Japan, however, clearly bucks the trend. While the country has more than 11 per cent of the world's broadband connections, it is producing only two per cent of global spam. This is despite the fact that 100Mbps fibre internet connections, which are especially attractive to spammers, are common in Japan.

The most likely reason for the correlation between broadband connections and spam output is the tendency for PCs on broadband lines to be taken over by malware and turned into so-called 'zombies', generating or relaying large quantities of spam.

Zombie PCs are believed to produce over 60 per cent of spam, according to security software firm Sophos, which gathered the data on the geographic origins of spam from its global network of spam detectors.  

Anecdotal evidence supports Japan's surprisingly low ranking in the spam charts, according to the Japanese subsidiary of Sophos, which develops software to combat virus infections, spam and malware.

"Despite our customer base in Japan expanding very significantly since we established the subsidiary in 2000, the number of calls to our support department regarding infections in general has fallen," said Alan Broderick, director of the company's Japanese office.

"If the proportion of machines in Japan which are infected at any one time is lower than in other countries it would certainly have a positive effect on the relay figures."