It is not Total Dissolved Solids (TDS) that cause problems in immune systems affected persons, it is the microbial content. The minute percentage of minerals in a given quantity such as a liter does not have any effect on the immune system of the person drinking the water. It is the microbial content that would affect someone. And, the potable water at WDW meets EPA guidelines.
So, to say that immune systems are affected by WDW water just is not correct. Here is what the CDC says: "Who might be immunocompromised or have a weakened immune system?
Examples of persons with weakened immune systems include those with AIDS; cancer and transplant patients who are taking certain immunosuppressive drugs; and those with inherited diseases that affect the immune system. The risk of developing severe disease may differ depending on each person's degree of immune suppression. Cryptosporidiosis (krip-toe-spo-rid-e-o-sis), is a diarrheal disease caused by a microscopic parasite, Cryptosporidium parvum, that can live in the intestine of humans and animals and is passed in the stool of an infected person or animal. Both the disease and the parasite are also known as "Crypto." WDW water does not have "Crypto".
Now, taste is a subjective matter to an individual. In potable water, taste is triggered by a combination of TDS in a given quantity of water. Purely objective measurements of TDS in WDW potable water by disinterested third parties would never fall into a "horrid" category, I submit.
BTW, the Brita pitcher filter does this:it is comprised of activated carbon and ion exchange resin. The filters activated carbon works to reduce chlorine. The ion exchange resin acts like a magnet, binding to its surface the heavy metal ions lead, copper, mercury, and cadmium, which have already been removed at WDW by the water company. So, the Brita pitcher filter removes chlorine at WDW. The chlorine is added to protect visitors.