IIRC this was after the house was broken into by zombies, and the father had taken a wound, which was bleeding. i.e. he was contaminated.
Someone upthread said that they wished that there had been more focusing on the biosciences types. I disagree. The only way that Brooks would have been able to make that work would have been with some really, really involved technobabble about what was going on on both a cellular and organism level. And the chances of him being able to make it convincing seem slight.
I thought WWZ was a better book for leaving the science behind what was going on off screen, where the details wouldn’t encourage the reader to think of why this, that or the other detail were impossible.
My favorite vignette was the interview with the filmmaker, and his discussion about how people need lies.