Computer illustration of T-bacteriophages, or phages, leaving a bacterium. Phages are viruses that infect bacteria. T-phages consist of an icosahedral (20-sided) head, which contains the genetic material (either DNA or RNA), and a thick tail with several bent tail fibres. The tail is used to inject the genetic material into the host cell to infect it. The phage then uses the bacterium's genetic machinery to replicate itself. When a sufficient number have been produced the phages exit the cell by lysis, a process that kills the cell.