The Katadyn Cyst Filter attaches to the middle of your Exstream ViruStat microbial water purification cartridge and can be replaced repeatedly throughout the life of the ViruStat system if needed. It removes pathogens such as protozoan cysts (Giardia and Cryptosporidium) and particulates larger than 1 micron. A must for extended travel in regions with questionable water quality.A must when traveling to regions with questionable water quality. Attaches to the middle of your Extream ViruStat microbial water purification cartridge. Can be replaced repeatedly over the life of your ViruStat system. Removes Giardia and Cryptosporidium, and particulates larger than 1 micron. Package of two.