Why is Ellie immune in 'The Last Of Us'?

HBO’s 'The Last Of Us' faithfully adapts the acclaimed video game into a prestige post-apocalyptic drama series.

Ellie is immune, what would her brain x-rays look like? We see X-rays of infected in-game, but obviously, hers did not spread, so what would the X-ray look like? I think either

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How is Ellie immune in The Last of Us?, TV & Radio, Showbiz & TV