Ok?

It could just be for gameplay reasons

I believe it's because she is in fact not infected. She is immune to the cordyceps. Yes you would think she carrys the infection in her and that she just doesn't change. SPOILER AHEAD: She cannot have the cordyceps in her due to the fact the fireflies want to do surgery on her organs and blood to find a cure and if her blood or organs were infected it would in fact infect whoever tries to use it for an antidote!

Another thing: why does Ellie die after being bitten when she is immune?

Well the infected do always bite her neck, so she could bleed out from that

Oh yeah

Ellie isn't infected, she's immune... What you talking about?

She’s infected but immune to the effects. She does have the cordyceps in her but it is mutated. A cure could only be made by infected blood or the fungus being engineered to cure people.

Well if a bloater pick Ellie of course she dies no? Logic

Cuz they're maaaaaddddd