A general explanation of why wastewater is used for tracking viral activity. This is meant to be a simple "explain it like I'm 5" kind of explanation, not a deep dive into the science. --- When a person is sick with a virus, that virus replicates as it spreads through your body. As your immune system works to kill it, you start to shed the virus. This ends up in your waste, just as food does, and continues until the virus is flushed from your system (which may be days or weeks after you start feeling better, and may even start days before you begin to first feel sick). Wastewater treatment plants sample wastewater to analyze health conditions in the community, checking possibly for COVID, flu, etc. From that sample, they can tell what proportion of that sample represents shed COVID viruses. That information is reported by the lab and tracked. Based on that, it's possible to tell if the proportions appear to grow or shrink over time. Not every bit of wastewater is tested, and not every person will shed the same amount, but it gives an idea as to whether there seems to be more or less COVID virus spreading around than there was a week ago, a month ago, 3 months ago, etc. Based on how that's changing, and based on what was considered low over time, we can get a sense as to how quickly it's also changing. From this, it's possible to calculate a Wastewater Viral Activity Level, which tells us if activity is very low, low, moderate, high, or very high. From those calculations, we currently know that the sampled parts of Chico and Oroville have gone from a consistent very low/low/moderate several months ago to a consistent high/very high over the past 1-2 months.