155. This Court has established that the right to life plays a fundamental role in the American Convention, because it is essential for the exercise of the other rights. 179 Compliance with Article 4, related to Article 1(1) of the American Convention, supposes not only that no one may be arbitrarily deprived of their life (negative obligation), but also requires States to take all appropriate steps to protect and preserve the right to life (positive obligation), in keeping with the obligation to ensure the full and free exercise of the rights of all persons subject to their jurisdiction. 180 Consequently, States have the obligation to create the required conditions to ensure that no violations of this inalienable right occur and, in particular, the duty to prevent their agents from violating it.181 156. The Court has indicated that in order to determine the international responsibility of a State in cases of death in a medical context, the following elements must be proved: (a) that, by act or omission, a patient is denied access to health care in situations of medical emergency, or to essential medical treatment, even though the risk to the patient’s life entailed by this denial is foreseeable; (b) serious medical negligence,182 and (c) the existence of a causal nexus between the act that has been proved and the harm suffered by the patient.183 When the attribution of responsibility stems from an omission, it is necessary to verify the probability that the omitted conduct would have ended the causal process that resulted in the harm. Such verifications must take into consideration whether the person concerned may be in a situation of special vulnerability 184 and, if so, the measures taken to respond to this situation.185 157. In this case, first, the Court notes that, of the 49 presumed victims, 12 are now deceased from opportunistic diseases. The Court recalls that opportunistic diseases are those infections or neoplasm that take advantage of a weakened immune system, and can cause the death of the person who contracts them. As mentioned previously (supra para. 37), antiretroviral therapy offers the best chance for effective suppression of the virus, recovery of the immune system and clinical benefits. Expert witness Boza Cordero stated that, when a patient is taking antiretroviral drugs appropriately, there is no reason for opportunistic diseases to occur; thus, if an opportunistic disease appears, it is because the patient is not taking the necessary medication, which constitutes a therapeutic failure.186 Cf. Case of the “Street Children” (Villagrán Morales et al.) v. Guatemala. Merits. Judgment of November 19, 1999. Series C No. 63, para. 144, and Case of Poblete Vilches et al. v. Chile. Merits, reparations and costs. Judgment of March 8, 2018. Series C No. 349, para. 145. 179 Cf. Case of Juan Humberto Sánchez v. Honduras. Preliminary objection, merits, reparations and costs. Judgment of June 7, 2003. Series C No. 99, para. 110, and Case of Poblete Vilches et al. v. Chile. Merits, reparations and costs. Judgment of March 8, 2018. Series C No. 349, para. 146. 180 Cf. Case of the “Street Children” (Villagrán Morales et al.) v. Guatemala. Merits. Judgment of November 19, 1999. Series C No. 63, para. 144, and Case of Poblete Vilches et al. v. Chile. Merits, reparations and costs. Judgment of March 8, 2018. Series C No. 349, para. 148 181 Cf. Case of Ximenes Lopes v. Brazil. Judgment of July 4, 2006. Series C No. 149, paras. 120 to 122, 146 and 150, and Case of Poblete Vilches et al. v. Chile. Merits, reparations and costs. Judgment of March 8, 2018. Series C No. 349, para. 148. 182 Case of Poblete Vilches et al. v. Chile. Merits, reparations and costs. Judgment of March 8, 2018. Series C No. 349, para. 148. 183 Cf. Case of the Xákmok Kásek Indigenous Community v. Paraguay. Merits, reparations and costs. Judgment of August 24, 2010. Series C No. 214, para. 227, and Case of Poblete Vilches et al. v. Chile. Merits, reparations and costs. Judgment of March 8, 2018. Series C No. 349, para. 148. 184 Cf. Case of Ximenes Lopes v. Brazil. Judgment of July 4, 2006. Series C No. 149, para. 125, and Case of Poblete Vilches et al. v. Chile. Merits, reparations and costs. Judgment of March 8, 2018. Series C No. 349, para. 148. 185 Expert opinion provided by Ricardo Boza Cordero at the public hearing held before the Court and written report on this opinion (merits file, folio 1458). 186 54