Monsters stay in one place for hours.

[Sexbad]

I'm not sure, but to reverse that, you may need to reinstall the game. It keeps your save games in another folder so they should be safe through the process. I suggest that if it's really that hard to get past the infected after he stops and waits for you, you lower the difficulty so you can run past it more quickly. At least, until you find some definitive fix.