An article published in the journal “Nature” reports a study concerning bacterial immune defenses of the CRISPR-Cas type, which could turn out to be a disadvantage against some types of viruses. A team of researchers analyzed the immune systems present in the genome of over 170,000 bacteria to try to understand the reactions against bacteriophage viruses. The conclusion that CRISPR antiviral systems are sometimes a disadvantage offers an explanation of why only a minority of bacteria have them in their DNA. It also offers new information on bacteria’s defense strategies viruses’ attack strategies with a sort of arms race between them.