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In the past, when there was no pasteurization, could making yogurt from milk lower the chance of getting infected by bovine tuberculosis (or other diseases from infected milk)?
For example, would yogurt cultures reduce the amount of "bad" bacteria or create a less hospitable environment for them?
In short, 'No.'
Yogurt, in and of itself, is the product of milk with specific strains of bacteria that are not particularly unique. Yogurt is just as hospitable to harmful bacteria as beneficial bacteria.
The two mechanisms which spring to my mind that would prevent infection by harmful bacteria in yogurt would be the following:
*The already dominant beneficial bacteria outcompete for the harmful bacteria, effectively limiting the capacity for harmful bacterial growth.
*The already present beneficial bacteria create extracellular products which damage harmful bacteria.
The second, if it happens at all, doesn't happen on a scale that I'm aware of. The first could happen, but I highly doubt it. It seems, to me, that it would be more likely to happen in cheese when most of the easy resources have been consumed, which in yogurt they haven't.
Yogurt, and all products stemming from milk, are inherently safer not because of any bacteria that take up residence, but because the mammary glands inside the animal producing the milk are effective filters for a variety of infections. Milk is a product that is constructed in the mammary glands, and the cells are selective about the output. Keep in mind, however, that it is by no means sterile. There are dozens of virii and diseases which can be imparted by breastmilk, and nursing women must adhere to guidelines concerning exposure to medications and diet. It's also possible for DDT and other compounds to become concentrated in breastmilk, resulting in harm to the child.
http://www.breastfeedingbasics.org/cgi-bin/deliver.cgi/content/Drugs/pre_pass.html
So, yes, while milk in and of itself is safer than other options, it is not risk free. A virus could easily infect the mammal or herd producing the milk for sale to humans, and only exhibit dangerous symptoms after the milk had been sold. This is why it's illegal to sell unpasteurized milk for human consumption in most U.S. states.
There have been studies regarding whether or not the bacterial load in yogurt (which would be the "Live and active cultures" part of yogurt - or the part where Jamie Lee Curtis talks about how you'll go back to pooping on schedule) helps protect the gut against the proliferation of certain pathogenic bacterias like C. difficile when we've otherwise wiped out the gut's natural flora - say with antibiotics.
So far, the results from that have been middling and inconclusive. And honestly, that's the best shot the cultures in yogurt have of showing a good effect.
In the scenario you ask about, you'd be trying to protect against diseases that haven't been handed a competitive advantage in the form of an obliterated gut flora, and have added in the risk of introducing pathogenic bacteria from the milk itself. A staggering proportion of milk-related food-born illness in the U.S. is due to unpasteurized milk. I'd say the chances of that being net-positive as slim, and more importantly I don't think you could slip that study past an IRB - at least in the U.S.
From Nico's question:
http://wwwnc.cdc.gov/eid/article/18/3/pdfs/11-1370.pdf Would be a decent place to start. Dairy is, admittedly, responsible for a small number of food born outbreaks overall - various fruits and vegetables, chicken and beef are still the top causes.
The issue is in how many outbreaks are caused by raw milk given how little of it is consumed in the U.S. The study I link above looks at known dairy outbreaks from 1993-2006. Of the 121 outbreaks, 60% of them were from unpasturized products, with a ~2 to 1 split between milk and cheese. All of them were caused by bacteria (addressing the idea that it might be a viral pathogen that wouldn't have been deactivated anyway).
The problem is though that many states prohibit the sale of raw milk products, and its relatively rare. If you weight the incidence of disease according to consumption patterns, one finds that the incidence per unit of dairy consumed is ~150x greater in non-pasturized products.
So while the overall numbers are not huge (though far from zero, and I've seen people consider lesser things public health emergencies), for such a rare product it's a big deal.
I've seen no suggestion that the patterns in Europe are different. For example (in Finnish) from this year: http://www.vsshp.fi/fi/7273/58327/ from this summer. But only the really massive outbreaks catch major media attention.
