They use the chlorine tablets to disinfect the water from springs, streams, rivers, etc.
Trekking and impure water
Impure water often contains bacteria, viruses, and parasites.
The most common cause of stomach infection on a trek is infection by E. coli bacteria.
Dysentery (diarrhea with blood) is also common.
It can be caused by bacteria, viruses and, protozoa.
Protozoa are by far the most feared and most common causes of water borne infection during a trek.
Many "chlorine" tablets contain chlorine dioxide (
Chlorine dioxide kills a wide variety of microorganisms in water including the parasites Giardia
(From Centers for Disease Control and Prevention)
How chlorine dioxide works
Chlorine dioxide is an oxidizing agent.
It moves through the microorganism’s cell walls and attacks amino acids in the cell’s cytoplasm, killing the microorganism.