As a general rule in life there is always a bigger fish – for every predator there is a bigger one lurking that is ready to eat it. However it is also worth remembering that there is usually always a smaller fish as well; for every small irritating parasite there is something that can infect it. Bacteria are no exception.
Bacteriophages are viruses that infect bacteria. They come in two different flavours; latent phages, which incorporate themselves into the bacterial genome and lie in wait, and lytic phages which infect bacteria, take over the cell, fill it with their offspring and then all burst out of it like the baby alien in Alien. While latent phages are fascinating for geneticists (not least because they provide a way of getting DNA into a bacteria) lytic phages are even more fascinating for people trying to kill bacteria, as they provide a way of destroying the cells.