A Grayl was recommended to me by someone who used one trekking in Nepal for 2 weeks. I’ve used it twice on the GDMBR and have been very happy with it. Their literature says it filters virus as well as bacteria though I’ve been told that is impossible. I’m not one to know. One of my Sawyer toting companions started borrowing my Grayl because it is so convenient. Available at REI. My wife and I have also used a battery operated steripen for various travels. The steripen has always turned on as it is supposed to but whether it does anything besides making me think I can now drink the water, I don’t know. To me it all seems a bit like voodoo.