I can firmly assert that it was NOT suggested that medical officer Ellis was immune. His arm was amputated immediately after suffering the bite, this is a common trope in zombie lore. In fact, a popular character on the show "The Walking Dead" was saved by having his leg amputated immediately after suffering a bite. In both cases, the infection was physically removed before it could spread to the rest of the body.
Additionally, his amputation became severly infected in the common way that one would imagine such a wound to become infected in a non-sterile environment (i.e. having nothing to do with the cordyceps fungus, but simply having to do with ambient bacteria in the environment). His fever became dire enough for him to behave irrationally, ultimately resulting in him murdering his savior in a bout of paranoia.
In the end, he simply bled out due to the fact that, in a later battle with infected (one in which he is not bitten again) he rips open his sutures and, as a result of being unable to close them himself, bleeds to death.
Also, a side note regarding immunity in other individuals:
It is in no point during the game stated that there are any other known cases of immunity, and this is most certainly NEVER mentioned by Marlene. So why is this floating around the internet?
This idea comes from "Surgeon's Recorder" in the mission "Firefly Lab" (Again, not from Marlene). Many have misunderstood the surgeon's monologue. This is understandable due to a good deal of medical jargon, thus I have quoted a gentleman from another forum below, his explanation is sound. It is the second italicized statement which has caused the confusion:
"The girl's infection is like nothing I've ever seen. The cause of her immunity is uncertain.
See that? The doctor has never seen a case like this and has no idea why she is immune.
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.
Translation: Like all other non-immune infected people, the levels of the fungus are high in Ellie's blood and brain/spinal fluid.
Blood cultures taken from the patient rapidly grow Cordyceps in fungal-media in the lab...
Translation: We can take a blood sample, put it on a petri dish in the lab and it will grow. Clearly, the fungus in her body is alive and has the ability to grow rapidly.
...however white blood cell lines, including percentages and absolute-counts, are completely normal.
Translation: Even with all these signs of infection, her white blood cell levels are normal and not elevated as they would be if her body detected an infection and were trying to fight it off.
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.
Translation: Cytokines, real biomolecules that help initiate the human body's inflammatory response to infection, are not elevated in Ellie. Again, her body doesn't seem to feel like it is infected nor is it trying to fight it off.
This all goes back to the doctor's previous statement of the cause of her immunity being uncertain.. If she clearly has the fungus in her body, but her body's infection defenses are completely normal, why is she still immune?"
So could there be other immune people? Of course! Is it stated that this is the case at any point during the game? Absolutely not. Unfortunately, Joel was lying through his teeth at the end, and the morality of his choice to save Ellie is highly questionable.
But these immune people were all different from Ellie.
Don't know what that means though...
How would you know? Again, it's been awhile since i've played this game but there is no proof that the Fireflies would be able to make a cure from Ellie.