A strain is a genetic variant or subtype of a microorganism, that is to say a virus or bacterium or fungus. For example, a "flu strain" is a certain biological form of the influenza or "flu" virus. Properly speaking, bacteria, archaea and viruses do not have species, because they do not have eukaryote-type sexual reproduction. Bacteria do exchange DNA, but they may exchange it between different kinds of bacteria, which shows their difference from eukaryotes. Strains are therefore an essential part of bacterial identification. 