Milk that has gone past its expiration date begins to develop bacteria that are sometimes harmful to the human body, leading to food poisoning if consumed. According to MedlinePlus, food poisoning has any or all of the following symptoms: headache, abdominal cramps, nausea, vomiting, weakness, fever, chills, diarrhea and blood in the stool.
After a certain period of time, milk starts to develop a bacteria called Streptococcus lactis. While this bacteria is often deliberately added to milk under controlled circumstances to make products such as sour cream and buttermilk, when left to its own devices, this normally beneficial bacteria consumes the milk and leaves behind waste materials that negatively affect the food. These waste materials lead to food poisoning in humans. Raw, unpasteurized milk is more likely to contain these pathogens.
Store milk in sealed containers, and refrigerate it properly. Use a thermometer in the refrigerator to check that the refrigerator's temperature stays below 40 degrees Fahrenheit. Additionally, pay attention to where the milk is placed. The upper shelves of a refrigerator tend to have the most consistent temperature; the door shelves are the warmest spot. When properly stored, pasteurized milk has a shelf life of roughly two weeks. Regardless, if the milk it has an odd smell or unusual texture, discard it.