British researchers have found that the drug resistance of some strains of HIV (Human Immunodeficiency Virus) is definitive. A total of 550 cases have been analyzed in which Birmingham's drug treatment has failed and very high levels of hiv-resistant virus have been measured in all of them. HIV achieves resistance thanks to mutations, and although the strain disappears when leaving treatment, when starting to take the same drug it returns to appear faster and in greater quantity than the previous one. The resistant virus appears to remain in the lymph nodes. The discovery has largely culminated in the hope of recycling drug treatments.