Book 30, Question 113
How big are germs, and how do filters catch them?
Germs are very tiny.
Some parasites in lake water are about 10 microns across, a micron being a thousandth of a millimeter. Bacteria are about 1 micron, and viruses can be 100 times smaller. A camping filter has pores smaller than the germs it aims to catch, but viruses may slip through, so people often add other steps. A grown-up should handle any camp filter.