Bacteriophages attacking bacteria. Illustration of bacteriophage viruses (blue) attacking bacteria (yellow). Each virion has a protein head containing DNA (deoxyribonucleic acid, spirals) genetic material and a contractile tail (shaft) with tail fibres (legs). The virion in the foreground is injecting its viral DNA into a bacterium. Once within the cell, the DNA will take over, effectively turning the bacterium into a virus-making machine. The red molecules are CAS enzymes, part of an adaptive immune system that protects bacteria from infection by recognising viral DNA and destroying it.