They could even be useful to medicine. Viruses are a kind of parasite, and many of them use bacteria as hosts. So if a person has a bacterial infection, the right virus could be used to treat it – rather than standard antibiotics, which have been massively over-used. At the moment such "phage therapy" is only really used in Russia, Poland and Georgia, but other countries are now taking it more seriously. The US National Institute of Allergy and Infectious Diseases recently announced plans to use it to combat antibiotic-resistant bacteria .