The contents of this particular recorder shed some light on Joel's story to Ellie in the game's epilogue. It states that Ellie's immunity is an anomaly that has never been seen before and that the Fireflies have experimented on other infected subjects, albeit ones with fungal growth that would cause aggression and elevated immune responses. The recorder is branded as a PCM Recording MP3 recorder.
Note: All punctuation and capitalization are typed up as it is heard in the game.
April 28th. Marlene was right. The girl's infection is like nothing I've ever seen. The cause of her immunity is uncertain. As we've seen in all past cases, the antigenic titers of the patient's Cordyceps remain high in both the serum and the cerebrospinal fluid. Blood cultures taken from the patient rapidly grow Cordyceps in fungal-media in the lab... however white blood cell lines, including percentages and absolute-counts, are completely normal. There is no elevation of pro-inflammatory cytokines, and an MRI of the brain shows no evidence of fungal-growth in the limbic regions, which would normally accompany the prodrome of aggression in infected patients.
We must find a way to replicate this state under laboratory conditions. We're about to hit a milestone in human history equal to the discovery of penicillin. After years of wandering in circles, we're about to come home, make a difference, and bring the human race back into control of its own destiny. All of our sacrifices and the hundreds of men and women who've bled for this cause, or worse, will not be in vain.
The contents of this particular recorder shed some light on Joel's story to Ellie in the game's epilogue. The surgeon mentions "past cases", which essentially means that they have experimented on other people who were infected but were unable to find a cure, but notes that Ellie's immunity may be the key to creating a vaccine.