Bacteriophages are viruses that only infect bacteria. Infection by tailed phages starts when specialized adsorption structures, such as fibers or spikes, bind to specific surface molecules or capsules on their target bacteria. Phage are the most abundant biological form on earth and have been estimated to outnumber their bacterial hosts by at least an order of magnitude. A bacterial population may also survive a phage attack if the specific cells infected by phage undergo an abortive infection; that is, the infected cells die before the phage have had the time to reproduce. With the advent of widespread DNA sequencing of many genomes, another mechanism of bacterial resistance to phage infection has been recognized. Bacteriophage package their genomes into phage capsids. Thus, one can argue that a bacterial sensor system of a low-salt environment activates synthesis of cholera toxin, and the profuse resultant diarrhea facilitates the release and spread of the bacteria from the affected individual.