Large viruses may have sophisticated immune systems MEHAU KULYK/SCIENCE PHOTO LIBRARY

The mimivirus is so enormous it has its own kind of CRISPR-like immune system to defend against the smaller viruses that attack it. A team in France has confirmed how it works by transferring the entire system to a bacterium and tweaking it to destroy a different target.

While CRISPR has become famous as a tool geneticists can use for editing genomes, it evolved in bacteria as a way of defending against viruses. The bacteria “cannibalise” bits of DNA from the viruses that attack them and add this …