Improve the water quality and filtration time of your MyBottle and Exstream personal water purification systems with the replacement Katadyn Exstream Virustat Kit. Change the filter when water doesn't taste as clean as you like or the filtration process slows, which occurs after five months of moderate use. The three part kit features a replacement pre filter, cyst filter, and ViruStat cartridge that works together to remove over 99. 9% of waterborne cysts and protozoa Giardia and Cryptosporidium and also kills over 99. 99% of waterborne viruses.