Absolutely. Which is why I was so horrified when I was looking for instructions on how to make yogurt and read one woman's method of heating raw milk to 120 degrees F and then adding the culture. Pathogens love those conditions!
You probably also know that testing for TB is common & that bacteria CAN be present in milk before it is expressed, but that the vast number of current horror anecdotes is of bacteria that are introduced after the milking. Unlike commercial dairies, home dairies filter the milk immediately after milking (within 5 minutes) & immediately cool it.
Any cow that is sick, even if it doesn't have symptoms, can shed the pathogens in their milk. And the pathogen count can be low enough that it does not show up in tests, but is still high enough to cause illness. Furthermore, given the proximity of the cow's udder and anus, I'd *love* to know how fecal bacteria can be kept out of the milk...they'll poop anywhere, any time, even during milking. I'd also like to know exactly what kind of filtering is supposedly occurring at the "home" dairies as opposed to the "commercial" dairies. Is the milk filtered through a 0.2 micrometer pore-size membrane, which removes (most) bacteria? Or is the filter only designed to remove fairly large particulates, which does nothing to remove bacteria? Commercial dairies do, in fact, cool the milk immediately upon milking. I actually do not expect the equipment in a small "home" dairy to be as sophisticated as that in a commercial dairy; a small dairy simply does not have the financial means to acquire the most technologically advanced equipment.
Commercial dairies put their milk into large tanks that can have an inch or so of green scum floating on top. But that seems to be ok, because eventually it is filtered out & heated to 160 degrees. The milk we buy at the store will last about 10 -14 days. Then it rots. The milk we use from our cows will last about 14 days & then it sours & can still be used for baking, etc.
I'd love to know the source of that green scum story; no doubt, the USDA/FDA/whatever other regulatory agencies would love to know about that dairy, as well. Considering that dairies are required to sanitize their equipment after every use, I think that dairy would be shut down in a heartbeat. As for how long milk lasts--the pasteurization process does not kill the bacteria responsible for decay, it only kills the pathogens. If you want milk that won't decay, you need to buy the UHT processed milk. It doesn't taste very good, IMO, so I don't buy it. If you want to use soured milk for baking, soured pasteurized milk is just as good for that purpose.
In a healthy cow, pathogens must be introduced from an outside source & a speck of dried manure, while it could contain LOTS of bacteria, likely will have no pathogens. Our milk is tested. We got a new cow & before her milk was consumed, we tested it. It had a high staph infection. She is used to feed calves rather than humans & is kept separate from the milk cow. She is also being treated with the hope the infection can be cleared up. So far only one quarter remains infected.
We use an isotropic filter. No, it won’t filter out individual bacteria, but our milk is not homogenized, either. But it lasts longer than store-bought milk. Perhaps the natural enzymes hold some of that bacteria in check.
I don’t know about other dairies, but most of our milk goes into quart or half-gallon containers which allows for much quicker cooling than a larger tank.
I would like to recall who told me the story about the scum, too.