It's not always obvious when water damage has escalated into a biohazard situation, but recognizing the signs is crucial for your safety. Generally, if the water comes from an unsanitary source, like a sewage backup, overflowing toilet with fecal matter, or floodwaters that have mixed with ground pollutants, you're looking at a biohazard. This is often referred to as 'black water' and is teeming with bacteria, viruses, and other harmful pathogens.
Even 'gray water' from an overflowing washing machine or dishwasher, if left untreated for too long, can degrade into a black water situation.