If the yolk of an egg is the primary objective for microorganisms since it has a great quantity of nutrients for a successful reproduction, in the case of the milk all of it is interesting for the same reason. That's why it's necessary to take extra care to prevent potential contamination with microorganisms, singularly those hazardous by nature.
As always, when the user has doubts, boiling is maybe the best solution instead of despising.
In this article the focus is in the user side, not in the production, but please do not forget that there's the rare possibility of inherent contamination. Normally, it would present observable signs for some way.
The first advice is to avoid direct contact with air as much as possible because this is a primary source of microorganisms. Another really important factor is temperature, being warm toward hot the most optimal for the development of bacteria.
The user must note that the process of decontamination varies, normally UHT (Ultra High Temperature) or pasteurization. UHT is using 130ºC and pasteurization 72-76ºC, creating an obvious difference in terms of nutrition, but also in proclivity to be contaminated.
Some insects carry microorganisms, so it's a good idea to not let them fly near or touch these materials that will be in contact with milk (or other aliments).
Another problem appears when the milk is stored in enamelware that is not completely clean. Dirt could mean presence of microorganisms, and like it was said early, milk is perfect for providing life support so add temperature and you will have more complexity. Some bacteria are capable of creating health issues, and they do right now statistically in a big percentage of the population. With care, their path will be harder!