For this achievement, every playable character must be bitten by an infected person / werewolf. It does not matter if the character goes on to die afterwards and the achievement will unlock after the final character is infected.
I recommend doing this as it's own playthrough. Don't breath events can not be put on AUTO for this achievement.
Max, Laura and Nick will always be infected in the story so you do not need to worry about those 3 characters for this achievement.
Chapter 1 - Break into the cabin room as Abi and Emma.
Chapter 3 - To get Abi infected, choose to help Nick after he is attacked, and progress until Abi is hiding behind a rock. Fail the Don't breath event and she will get infected. Later in the Chapter, when Jacob is caught in a noose by Bobby, throw dirt at him.
Chapter 4 - As Emma on the island, go into the treehouse and search the bag. Use the taser and progress to run away passing the QTEs. When back in the treehouse, choose Block Door, then use the trapdoor. Pass the first QTE and then fail the QTE to push the cabinet then pass the next QTE to taser the werewolf again. Make sure Emma survives.
Chapter 5 - Dylan will automatically get bitten here if Abi and Emma broke into the cabin in chapter 1. Do not chop off Dylan's arm.
Chapter 6 - As Jacob, when he is first attacked, fail the first QTE and he will get bitten. Pass the rest of the QTEs and progress. Later as Emma, when you come across the two hunters you can get bitten again as a precaution. Choose to Run instead of hide/wait.
Chapter 9 - As Ryan accept Laura's bite (see "Phlebotomy"). Later in the scrapyard, choose to slam the car when the werewolf is on it and fail the QTE. Kaitlyn will be bitten and the achievement should unlock if everything has been done correctly.
Credit to ThePugHybrid for their
guide.