In the beggining of the game you learn that Ellie is immune to the CBI by showing a bite on her arm that is about 1 week old. Near the very end of the main story wehn you bring Ellie to the hospital Marlene implies that the only way to find how Ellie is immune to CBI is that she needs to undergo braiin surgery. But the way Joel reacted when he heard this had me confused, probably because he thought the surgery would kill her. Marlene also thought the surgery would kill her, but right now most operations on the brain donk't result in the patients death. Maybe the suegeons wouldn't be able to stop Ellie's brain fluids form leaking, or they didn't kknow her blood type so they wouldn't be able to give her a transfusion. Maybe because the fungus grows in and around the brain the surgeneos may have to damage Ellie's brain in order to extract the fungus to study it. Then i thought, well the fungus most likey travels to the brain by the host's blood stream maybe some of the fungus was still in Ellie's blood stream, if this were the case the surgeons would've been able to take some of the fungus directly from her blood or extract some of her blood and filter out the fungus (I don't know how that would work) and study that way. One more thing, what if it isn't the type of mutation of hte virus Ellie has? what if it's just her immune system that is resistant to the fungus, this wold be another reason to studie Ellies blood.