Bacteria
Bacteria are microscopically small, single-cell organisms. In congenial conditions they can multiply very rapidly. All they need to grow is water, food and the right ambient temperature. Bacteria spoil the food in which they grow and can make people ill. Certain bacteria have a preference for certain types of food. Milk is the ideal habitat for particular types of bacteria. If they get a chance to multiply, the milk literally goes sour. You can taste this and also see it, because the milk curdles and separates.
Read more about the shelf life of milk and dairy products