You can put the water in a clear plastic container (such as an empty 2 liter bottle), shake it up, and leave it out in the sun for a few hours to kill the nasties inside if I recall correctly, but I've never tried it and I think it requires a warm/hot day. I don't know exactly how it works... I guess shaking it up aerates the water and UV light and heat from the sun kills the things inside or something.

Or you can put a cup of contaminated water in a solar still. Instead of producing water from vegitation like traditional solar stills, this is supposed to evaporate the existing, contaminated water, have the steam condense on the plastic, and have this water drip down into a clean container. Again, requires some warmth I imagine.

I dunno, someone who knows more about it can probably clarify whether these methods would work or not. I don't know if these methods kill viruses and crypto...