You probably don't replace the filter frequently enough
Brita recommends that you change your water filter every 40 gallons. So let's see: your family drinks a gallon a day, over the last six months, plus water used for cooking, carry the 2, minus a week vacation time, and that comes to... who the hell even knows. The chipper green indicator light has been a fiery red Terminator eye for the last few weeks, so it's still good, right? Well, if you consider a dark, damp net that's been catching bacteria for the last four months to be "good," then sure.
Despite what most of us want to believe, Brita filters aren't designed to filter out bacteria or viruses. What's even scarier are the results of a study that compared the microbiological contamination of tap water to Brita filtered water. In some cases, the filtered water contained a whopping 10,000 times the bacteria colony count as tap water.
Granted, these were new Brita filters, so these findings probably don't apply to your aging filter... because it's probably much worse (bazinga!). Now, not only is your filter completely ineffective at filtering out actual dangers in your water, but it's also providing a romantic getaway for bacteria colonies. What's a thirsty girl to do?