What is the test for a prisoner to prove that a prison official was deliberately indifferent to serious threats to the inmate's safety?

To establish a violation of this duty, the prisoner must establish that prison officials were "deliberately indifferent" to serious threats to the inmate's safety. Id. at 834. To demonstrate that a prison official was deliberately indifferent to a serious threat to the inmate's safety, the prisoner must show that "the official [knew] of and disregard[ed] an excessive risk to inmate . . . safety; the official must both be aware of facts from which the inference could be drawn that a substantial risk of serious harm exists, and [the official] must also draw the inference." Id. at 837; Anderson v. County of Kern, 45 F.3d 1310, 1313 (9th Cir. 1995). However, to prove knowledge of the risk, the prisoner may rely on circumstantial evidence; in fact, the very obviousness of the risk may be sufficient to establish knowledge. Farmer, 511 U.S. at 842.