Viruses are accidentally infecting worms on victims’ computers, creating super-powered strains of hybrid software.

The monster malware spreads quicker than before, screws up systems worse than ever, and exposes private data in a way not even envisioned by the original virus writers.

A study by antivirus outfit BitDefender found 40,000 such “Frankenmalware samples” in a study of 10 million infected files in early January, or 0.4 per cent of malware strains sampled. These cybercrime chimeras pose a greater risk to infected users than standard malware, the Romanian antivirus firm warns.

“If you get one of these hybrids on your system, you could be facing financial troubles, computer problems, identity theft, and a wave of spam thrown in as a random bonus,” said Loredana Botezatu, the BitDefender analyst who carried out the study. “The advent of malware sandwiches throws a new twist into the world of malware. They spread more efficiently, and will become increasingly difficult to predict.”

BitDefender doesn’t have historical data to go on. Even so it posits that frankenmalware is likely to grow at the same rate as regular computer viruses, or about 17 per cent year on year.