Prevention of traveler's diarrhea falls into four broad categories: immunization, avoidance, nonpharmacological therapy, and antibiotic prophylaxis. For U.S. residents, immunization plays an almost insignificant role in the prevention of diarrhea, since vaccines are not yet commercially available in this country for the vast majority of causative agents, although several are currently under development or in the process.Avoidance of high-risk foods and drink is an oft-cited means of reducing the risk of traveler's diarrhea. Water should be boiled vigorously for at least 1 min before consuming, which will kill most pathogens. Care should be taken when one is traveling at altitudes higher than 6,562 feet (2,000 m) to boil water for at least 3 min due to the lower atmospheric pressure. Adding either tincture of iodine (5 drops/qt) or tetraglycine hydroperiodide tablets, or using iodinizing filters, is also an effective means of purifying water, although protozoan cysts are often halide resistant.