Even though there were some deviations from the storyline of the first game in the first season of The Last of Us, the plot of the first game was very faithfully followed, even with the very end of the season when the ninth episode was aired.
In spite of this, it turned out to be worthwhile for the die-hard fans of the games to follow the series, as a few important things were explained with the series. The flashbacks in the first two episodes provide insights into the origin of the virus, while the events leading up to the end of the third episode reveal a great mystery.
Initially, Ellie seems to be immune to the virus. How and why is this the case? This has been the subject of speculation for years, and now that it is being explained, fans can stop speculating. There was a sense of time travel in the beginning of the season finale of The Last of Us TV series.
There is a baby on the way for Anna Williams - Ellie Williams. After a chaotic birth, just before she cuts the umbilical cord, she is bitten by a creature infected with an infectious disease and, unfortunately, it is the end for her.
After Ellie appears, Marlene asks her to take care of her daughter until she asks her for a bullet in the forehead.
Anna and Marlene
Anna tells Marlene that the bite occurred after the birth and the cutting of the umbilical cord of Ellie, so that Marlene does not kill Ellie as well.
During the time of the bite, Ellie was physically connected to her mother, and it appears that this physical connection was what provided her with immunity. Marlene tells Joel the conclusion of the scientist fourteen years later just before the planned operation on Ellie's brain is scheduled to take place.
A theory has been put forth that "normal" cordyceps cannot be used to treat Ellie due to the fact that she already has one inside her that has been growing with her since she was born. A new question comes to mind now. Is it possible that there are other people who are immune to cordyceps as well? Have any other mothers been bitten during the birthing process? That theory will surely be tested by someone within Fireflies at some point in time.