The only sure thing is to boil the water before you drink it. I personally have used a Pur hiker microfilter for years. Not too expensive and the cartridges are pretty abundant in my area. The Extream filter bottle is supposed to filter out viruses but some on here may feel that it lacks sufficient capacity. The hiker can filter a couple hundred gallons before you need to replace the filter cartridge and the water will taste like you just dipped your lips in the stream