What is the most costly computer system infection in history?

The most expensive trojan horse in history is the Love Bug virus, which was launched in 2000. This infection triggered an estimated $5.5 billion in damage worldwide, making it the most expensive infection ever.  see this The Love Bug virus was a mass-mailing worm that infected over 45 million computer systems in simply a couple of days. The virus spread rapidly by emailing itself to everybody in the victim's address book.  check my reference When a computer system was infected, the virus would erase important files and spread itself to other computer systems. The Love Bug infection was eventually stopped by a group of antivirus researchers who produced a vaccine that could protect computer systems from the infection.