Do water purifying tablets kill viruses and Giardia?

It depends on the active chemistry. Chlorine dioxide tablets (such as Katadyn Micropur MP1 and Potable Aqua Chlorine Dioxide) are generally marketed as effective against bacteria, viruses, Giardia, and Cryptosporidium, though Crypto requires the longest contact time (often up to 4 hours). NaDCC/chlorine tablets like Aquatabs and iodine tablets handle bacteria and viruses well but are less reliable against Cryptosporidium. Always follow the stated dwell time on the package.