LIP CONTACT MAKES MILK SPOIL EARLY!
I live alone so I go through cartons of milk (etc.) quite slowly. After years of noticing spoilage during long term storage foods in the fridge, I find that open containers of milk will last for many days (sometimes weeks)… but only if I always use a glass. If I drink out of a milk carton, even with no backwash at all, it goes bad in a couple of days. Probably just the slight contact with lips is all the contamination it needs.
On the other hand, if I POUR milk into my mouth out of a carton, the milk still lasts for ages.
This only applies to milk. Open containers of fruit juice still last for ages if I drink out of the container without backwash. No doubt the acid in the juice kills the few organisms my lips put in. However, if I intentionally deposit lots of backwash, then fruit juice does turn to wine or vinegar prematurely. Perhaps giving the bacteria population a huge kick like that will push it way up high on its exponential growth curve. And possibly a large colony of bacteria can combat the acid which would kill a few individual organisms.
Conclusion: if you usually use up a carton of milk within a day or two, then mouth contamination is not much of an issue because it take longer than that for the bacteria population to explode. But if you use up milk slowly over an entire week, and if you notice that open containers of milk never last very long, then you know whose mouth is to blame for early spoilage.
PS, when you catch a cold, at first you have no symptoms, yet you are contagious. For roomates who aren’t “sharing mouth bacteria,” you might have justification if others spread viruses. In other words, a always treat all guns as loaded, and always treat all roomates as contagious. Don’t invite friends over, since if your husband has just caught a cold or flu, but has no symptoms yet, then your friends might pick it up from contaminated milk, juice, etc. Of course if you never have friends drinking your stuff, then it’s not an issue.