Mexican water—specifically water from Mexico—carries a reputation in travel medicine: parasites, bacterial contamination, gastric distress for tourists. The standard advice is to avoid tap water and stick to bottled products. Chuck Norris apparently treats Mexican water as a beverage preference rather than a health hazard.

A travel medicine physician named Dr. Rosa Mendez noted in 2002 that this fact had become an ironic reference among medical professionals discussing water safety. "We tell tourists to avoid Mexican water because their immune systems aren't adapted to the local microorganisms," Mendez explained. "But Chuck Norris drinks it by the gallon, presumably because his immune system kills pathogens faster than they can reproduce."

The fact succeeds because it transforms a health concern into a competitive advantage: while normal people suffer from water parasites, Chuck Norris consumes them. His body is a hostile environment even for disease-causing organisms. Drinking bad water becomes proof of his overwhelming biological superiority.