This is a unit I have used in the past for remote canoe trips into Algonquin Park.
http://www.purwaterfilter.com/purguidpursy.html. Quite effective in removing 99.9% of all impurities, including viruses.
Also remember, any illness that you would pick up ie: "beaver fever" ( a virus) will not show for several weeks after your trip, and by that time, you will have a difficult time remembering where you picked up the virus.
Always use boiled water to clean your dishes, rinsing or washing in the lake water could have the same effect as drinking unfiltered water.
John