Illustration of Escherichia coli bacteria (purple) being infected by a group of bacteriophage (phage) T7 viruses (spherical). T7 bacteriophages consist of a large DNA-containing head and a tail composed of a central sheath with several fibres (green). The fibres attach to the host cell surface, and the phage DNA is injected into the cell through the sheath. It instructs the host to build copies of the phage. When a sufficient number has been produced, the phages burst out of the cell, killing it. Phages are a possible therapy against multi-drug-resistant bacteria. The illustration is at atomic resolution and at scale.