When I started backpacking I always carried a small bottle of hand sanitizer to use on my hands after filtering water and using the bathroom. I’d repackage it from a big bottle of Purell into 1 or 2 oz polyethylene squeeze bottles and put it into a hip belt pocket for easy access. It made perfect sense. Purell also sells 2 oz travel-size bottles of hand sanitizer that you can use and reuse.
When you purify or filter a backcountry water source to remove viruses, bacteria, and protozoa, touch the doorknob on a privy door, or a trail register, your hands can get contaminated with foreign bacteria, viruses, and organisms that you don’t want to ingest. This can happen when you reach into someone else’s food bag to eat some gorp, rub your eyes, or pick your nose. While your body’s immune system does a pretty good job of fighting off infections, it’s not always successful.
For example, Norovirus is very prevalent in the hiking community, especially on more crowded trails where many hikers use the same hostels, outhouses, and shelters. Research has shown that it only requires 10 – 20 Norovirus particles to become infected, so taking precautions is probably warranted if you run into strangers or camp with them.
I stopped carrying hand sanitizer after a few years because I trained myself not to put my “dirty” hands where they could infect me. I also avoid trails and campsites that have a lot of people on them because I prefer solitude when I’m out backpacking and enjoy having more of a wilderness experience.