Water filters generally remove the larger contaminates, like the gardia cysts, but fail to remove the smaller bacteria and viruses. In the United States, a water filter will do the job. However, in other countries, people need to use water purifiers. The water purifiers look just like the water filters, but they filter out the much smaller contaminates, like bacteria and viruses. People who do field research in other countries need to use high quality water purifiers to clean their drinking water.
Water is heavy.
On a hot day in the field, you may drink 5 liters.