They are typically spread by humans and are most likely to occur in areas with significant human contact, particularly in developing countries. Overseas travelers especially should be sure to properly treat water to remove viruses. Boiling kills just about everything in the water. However, lighting up a stove every time you want a drink is inconvenient, and the weight of extra fuel on an extended trip makes this method very impractical. Boiling does not remove chemicals or reduce turbidity.
Chemical disinfection is a generally lightweight method, easy to use, inexpensive and effective.
But users should be aware of several disadvantages. Certain protozoan cysts, particularly Cryptosporidium, are remarkably resistant to iodine. The taste is strong and unpleasant if a neutralizer is not used. For health reasons, iodine tablets are not recommended for prolonged use. Some people, particularly those with thyroid conditions and pregnant or nursing women, should avoid iodine completely.