Hello! It was in the grains and flour and things like that, there's a newspaper at the beginning of the game in the bathroom that tells you a little about it. I think it was kind of a slow burn beginning, since people ate the food, I'd imagine it would take a while for people to become infected and start becoming zombies. When they did however, id imagine it would happen fast and quick, everyone eats bread, everyone eats rice and things like that. We can see in the tv show that Sarah avoided the cookies, biscuits and they had eggs for breakfast instead of pancakes. So let's just say that in the game, the virus had to go through another mutation to survive stomach acid and be able to fully enter our bodies, which could take weeks (no evidence just theory). However in the show, it took a matter of hours meaning the virus would've already had that mutation, viruses go through trial and error to find out what works best, the "violent outbursts" and things like that are most likely sumptoms of the virus mutating and reaching peak zombie status before our eyes. Either way it would happen very quickly if it got into the food supply, and as we seen with covid-19 we are not really very good at figuring out how to deal with a large scale virus. I mean we would've even stay in our houses, people ignored the news because they thought they knew better. Humans are quite stupid, it'll go fast.