Many companies are trying to prevent leakage of personal and classified information by building independent networks isolated from the Internet, as viruses are often transmitted via files attached to e-mail or those on the Internet. However, USB drives are often used on unspecified computers, so viruses can immediately spread.

Antivirus software maker Trend Micro Inc. has found that reported computer virus infections via USB flash memory drives more than doubled in September, Jiji Press learned.

Infections in the month with the Otorun worm, which propagates via removal drives such as USB drives, surged 140 percent from the previous month to 347 cases, Trend Micro said in a monthly survey report.