Travelers' diarrhea is the most common travel-related ailment. The cornerstone of prevention is food and water precautions, as outlined below. All travelers should bring along an antibiotic and an antidiarrheal drug to be started promptly if significant diarrhea occurs, defined as three or more loose stools in an 8-hour period or five or more loose stools in a 24-hour period, especially if associated with nausea, vomiting, cramps, fever or blood in the stool. A quinolone antibiotic is usually prescribed: either ciprofloxacin (Cipro)(PDF) 500 mg twice daily or levofloxacin (Levaquin) (PDF) 500 mg once daily for a total of three days. Quinolones are generally well-tolerated, but occasionally cause sun sensitivity and should not be given to children, pregnant women, or anyone with a history of quinolone allergy. Alternative regimens include a three day course of rifaximin (Xifaxan) 200 mg three times daily or azithromycin (Zithromax) 500 mg once daily. Rifaximin should not be used by those with fever or bloody stools and is not approved for pregnant women or those under age 12. Azithromycin should be avoided in those allergic to erythromycin or related antibiotics. An antidiarrheal drug such as loperamide (Imodium) or diphenoxylate (Lomotil) should be taken as needed to slow the frequency of stools, but not enough to stop the bowel movements completely. Diphenoxylate (Lomotil) and loperamide (Imodium) should not be given to children under age two.

Most cases of travelers' diarrhea are mild and do not require either antibiotics or antidiarrheal drugs. Adequate fluid intake is essential.

If diarrhea is severe or bloody, or if fever occurs with shaking chills, or if abdominal pain becomes marked, or if diarrhea persists for more than 72 hours, medical attention should be sought.

Though effective, antibiotics are not recommended prophylactically (i.e. to prevent diarrhea before it occurs) because of the risk of adverse effects, though this approach may be warranted in special situations, such as immunocompromised travelers.

The following are the recommended vaccinations for Mexico City.

Measles vaccine is recommended for any traveler born after 1956 who does not have either a history of two documented measles immunizations or a blood test showing immunity. A measles outbreak was reported from Distrito Federal, Estado de Mexico, and Hidalgo in early 2004. Although measles immunization is usually begun at age 12 months, consider giving an initial dose of measles vaccine to children between the ages of 6 and 11 months who will be traveling to Mexico. Many adults who had only one vaccination show immunity when tested and do not need the second vaccination. Those born before 1957 can be presumed to be immune to measles. Measles vaccine should not be given to pregnant or severely immunocompromised individuals.

Hepatitis A vaccine is recommended for all travelers over one year of age. It should be given at least two weeks (preferably four weeks or more) before departure. A booster should be given 6-12 months later to confer long-term immunity. Two vaccines are currently available in the United States: VAQTA (Merck and Co., Inc.) (PDF) and Havrix (GlaxoSmithKline) (PDF). Both are well-tolerated. Side-effects, which are generally mild, may include soreness at the injection site, headache, and malaise.

Older adults, immunocompromised persons, and those with chronic liver disease or other chronic medical conditions who have less than two weeks before departure should receive a single intramuscular dose of immune globulin (0.02 mL/kg) at a separate anatomic injection site in addition to the initial dose of vaccine. Travelers who are less than one year of age or allergic to a vaccine component should receive a single intramuscular dose of immune globulin (see hepatitis A for dosage) in the place of vaccine.

Typhoid vaccine is recommended for all travelers. It is generally given in an oral form (Vivotif Berna) consisting of four capsules taken on alternate days until completed. The capsules should be kept refrigerated and taken with cool liquid. Side-effects are uncommon and may include abdominal discomfort, nausea, rash or hives. The alternative is an injectable polysaccharide vaccine (Typhim Vi; Aventis Pasteur Inc.) (PDF), given as a single dose. Adverse reactions, which are uncommon, may include discomfort at the injection site, fever and headache. The oral vaccine is approved for travelers at least six years old, whereas the injectable vaccine is approved for those over age two. There are no data concerning the safety of typhoid vaccine during pregnancy. The injectable vaccine (Typhim Vi) is probably preferable to the oral vaccine in pregnant and immunocompromised travelers.

All travelers should be up-to-date on routine immunizations, including

• Tetanus-diphtheria vaccine (recommended for all travelers who have not received a tetanus-diphtheria immunization within the last 10 years.)
• Varicella (chickenpox) vaccine (recommended for any international traveler over one year of age who does not have either a history of documented chickenpox or a blood test showing immunity. Many people who believe they never had chickenpox show immunity when tested and do not need the vaccine. Varicella vaccine should not be given to pregnant or immunocompromised individuals.)

An outbreak of H1N1 influenza ("swine flu") was reported from Mexico in April 2009. The outbreak was caused by a previously unknown strain of influenza that contained a unique combination of swine, avian, and human influenza gene segments. Initial reports indicated a high fatality rate in previously healthy young adults and older children, raising concerns that a worldwide pandemic might occur, similar to 1918. However, subsequent data from Mexico, as well as experience from the United States and other countries, indicate the H1N1 strain from 2009 is not nearly as lethal as some people initially feared. As of June 29, a total of 8279 cases and 116 deaths had been reported. The states with the highest number of confirmed cases are the Federal District, Estado de Mexico, San Luis Potosi and Hidalgo. The majority of these have occurred in previously healthy young adults. New cases continue to be reported, but the outbreak appears to be coming under control. Schools and universities have been reopened.

The World Health Organization and the Centers for Disease Control do not recommend any travel restrictions at this time. To protect yourself from H1N1 influenza, wash your hands regularly and avoid close contact with anyone who is coughing or sneezing. Routine use of face masks is not recommended. The symptoms of H1N1 influenza include fever, cough, sore throat, body aches, headache, chills and fatigue, similar to seasonal influenza. Any traveler to Mexico who develops flu-like symptoms should immediately seek medical attention. Clinicians who suspect H1N1 influenza virus infections in humans should obtain a nasopharyngeal swab from the patient, place the swab in a viral transport medium, and contact their state or local health department to facilitate transport and timely diagnosis at a state public health laboratory. Empiric treatment with Tamiflu or Relenza should be considered for suspected cases. It is not anticipated that the influenza vaccine given in the winter of 2008-2009 will be protective against the novel H1N1 virus. For further information, go to the World Health Organization and the Centers for Disease Control.

An outbreak of measles was reported in May 2004 from the Federal District and the states of Mexico, Hidalgo, Campeche, and Coahuila, resulting in 64 cases as of May 17. Most cases occurred in those greater than 15 years of age, many of whom remain susceptible to measles because they never received a second dose of measles vaccine as children. All travelers born after 1956 should make sure they have had either two documented measles immunizations or a blood test showing measles immunity (see discussion above). Those born before 1957 are in general presumed to be immune to measles. .

HIV (human immunodeficiency virus) infection is reported, but travelers are not at risk unless they have unprotected sexual contacts or receive injections or blood transfusions.

Other infections include

• Hepatitis E
• Shigellosis
• Amebiasis
• Brucellosis

Do not drink tap water unless it has been boiled, filtered, or chemically disinfected. Do not drink unbottled beverages or drinks with ice. Do not eat fruits or vegetables unless they have been peeled or cooked. Avoid cooked foods that are no longer piping hot. Cooked foods that have been left at room temperature are particularly hazardous. Avoid unpasteurized milk and any products that might have been made from unpasteurized milk, such as ice cream. Avoid food and beverages obtained from street vendors. Do not eat raw or undercooked meat or fish, including ceviche. Some types of fish may contain poisonous biotoxins even when cooked. Barracuda in particular should never be eaten. Other fish that may contain toxins include red snapper, grouper, amberjack, sea bass, and a large number of tropical reef fish.

All travelers should bring along an antibiotic and an antidiarrheal drug to be started promptly if significant diarrhea occurs, defined as three or more loose stools in an 8-hour period or five or more loose stools in a 24-hour period, especially if accompanied by nausea, vomiting, cramps, fever or blood in the stool. Antibiotics which have been shown to be effective include ciprofloxacin (Cipro), levofloxacin (Levaquin), rifaximin (Xifaxan), or azithromycin (Zithromax). Either loperamide (Imodium) or diphenoxylate (Lomotil) should be taken in addition to the antibiotic to reduce diarrhea and prevent dehydration.

If diarrhea is severe or bloody, or if fever occurs with shaking chills, or if abdominal pain becomes marked, or if diarrhea persists for more than 72 hours, medical attention should be sought.

Bring adequate supplies of all medications in their original containers, clearly labeled. Carry a signed, dated letter from the primary physician describing all medical conditions and listing all medications, including generic names. If carrying syringes or needles, be sure to carry a physician's letter documenting their medical necessity.Pack all medications in hand luggage. Carry a duplicate supply in the checked luggage. If you wear glasses or contacts, bring an extra pair. If you have significant allergies or chronic medical problems, wear a medical alert bracelet.

Make sure your health insurance covers you for medical expenses abroad. If not, supplemental insurance for overseas coverage, including possible evacuation, should be seriously considered. If illness occurs while abroad, medical expenses including evacuation may run to tens of thousands of dollars. For a list of travel insurance and air ambulance companies, go to Medical Information for Americans Traveling Abroad on the U.S. State Department website. Bring your insurance card, claim forms, and any other relevant insurance documents. Before departure, determine whether your insurance plan will make payments directly to providers or reimburse you later for overseas health expenditures. The Medicare and Medicaid programs do not pay for medical services outside the United States.

Pack a personal medical kit, customized for your trip (see description). Take appropriate measures to prevent motion sickness and jet lag, discussed elsewhere. On long flights, be sure to walk around the cabin, contract your leg muscles periodically, and drink plenty of fluids to prevent blood clots in the legs. For those at high risk for blood clots, consider wearing compression stockings.

Avoid contact with stray dogs and other animals. If an animal bites or scratches you, clean the wound with large amounts of soap and water and contact local health authorities immediately. Wear sun block regularly when needed. Use condoms for all sexual encounters. Ride only in motor vehicles with seat belts. Do not ride on motorcycles.

Air pollution reached unacceptable levels in Mexico City in 1998 due to smoke from forest fires in southern Mexico and low rainfall. Pollution is typically most severe from December to May. Travelers with respiratory or cardiac conditions and those who are elderly or extremely young are at greatest risk for complications from air pollution, which may include cough, difficulty breathing, wheezing, or chest pain. The risk may be minimized by staying indoors, avoiding outdoor exercise, and drinking plenty of fluids.

Up-to-date information concerning air quality in Mexico City and the environs is available from the Climate Institute.

Bring adequate supplies of all medications in their original containers, clearly labeled. Carry a signed, dated letter from the primary physician describing all medical conditions and listing all medications, including generic names. If carrying syringes or needles, be sure to carry a physician's letter documenting their medical necessity.Pack all medications in hand luggage. Carry a duplicate supply in the checked luggage. If you wear glasses or contacts, bring an extra pair. If you have significant allergies or chronic medical problems, wear a medical alert bracelet.

Make sure your health insurance covers you for medical expenses abroad. If not, supplemental insurance for overseas coverage, including possible evacuation, should be seriously considered. If illness occurs while abroad, medical expenses including evacuation may run to tens of thousands of dollars. For a list of travel insurance and air ambulance companies, go to Medical Information for Americans Traveling Abroad on the U.S. State Department website. Bring your insurance card, claim forms, and any other relevant insurance documents. Before departure, determine whether your insurance plan will make payments directly to providers or reimburse you later for overseas health expenditures. The Medicare and Medicaid programs do not pay for medical services outside the United States.

Pack a personal medical kit, customized for your trip (see description). Take appropriate measures to prevent motion sickness and jet lag, discussed elsewhere. On long flights, be sure to walk around the cabin, contract your leg muscles periodically, and drink plenty of fluids to prevent blood clots in the legs. For those at high risk for blood clots, consider wearing compression stockings.

Avoid contact with stray dogs and other animals. If an animal bites or scratches you, clean the wound with large amounts of soap and water and contact local health authorities immediately. Wear sun block regularly when needed. Use condoms for all sexual encounters. Ride only in motor vehicles with seat belts. Do not ride on motorcycles.

For an ambulance in Mexico, call 060.

For an online list of physicians and hospitals in Mexico City, go to the U.S. Embassy website.

Adequate medical care can be found in all major cities. Excellent health facilities are available in Mexico City. Care in more remote areas is limited. Standards of medical training, patient care and business practices vary greatly among medical facilities in beach resorts throughout Mexico. In addition to other publicly available information, Americans may consult the U.S. Embassy's web site or the U.S. Embassy, a consulate or consular agency prior to seeking medical attention.

Before you leave, make sure you have the names and contact information for physicians, clinics, and hospitals where you can obtain emergency medical care if needed (see the U.S. Embassy website).

All children should be up-to-date on routine childhood immunizations, as recommended by the American Academy of Pediatrics. Children who are 12 months or older should receive a total of 2 doses of MMR (measles-mumps-rubella) vaccine, separated by at least 28 days, before international travel. Children between the ages of 6 and 11 months should be given a single dose of measles vaccine. MMR vaccine may be given if measles vaccine is not available, though immunization against mumps and rubella is not necessary before age one unless visiting a country where an outbreak is in progress. Children less than one year of age may also need to receive other immunizations ahead of schedule (see the accelerated immunization schedule).

When traveling with young children, be particularly careful about what you allow them to eat and drink (see food and water precautions), because diarrhea can be especially dangerous in this age group and because the vaccines for hepatitis A and typhoid fever, which are transmitted by contaminated food and water, are not approved for children under age two. Baby foods and cows' milk may not be available in developing nations. Only commercially bottled milk with a printed expiration date should be used. Young children should be kept well-hydrated and protected from the sun at all times.

Be sure to pack a medical kit when traveling with children. In addition to the items listed for adults, bring along plenty of disposable diapers, cream for diaper rash, oral replacement salts, and appropriate antibiotics for common childhood infections, such as middle ear infections.

International travel should be avoided by pregnant women with underlying medical conditions, such as diabetes or high blood pressure, or a history of complications during previous pregnancies, such as miscarriage or premature labor. For pregnant women in good health, the second trimester (18–24 weeks) is probably the safest time to go abroad and the third trimester the least safe, since it's far better not to have to deliver in a foreign country.

Before departure, make sure you have the names and contact information for physicians, clinics, and hospitals where you can obtain emergency obstetric care if necessary (see the U.S. Embassy website). In general, pregnant women should avoid traveling to countries which do not have modern facilities for the management of premature labor and other complications of pregnancy.

Strict attention to food and water precautions is especially important for the pregnant traveler because some infections, such as listeriosis, have grave consequences for the developing fetus. Additionally, many of the medications used to treat travelers' diarrhea may not be given during pregnancy. Quinolone antibiotics, such as ciprofloxacin (Cipro) and levofloxacin (Levaquin), should not be given because of concern they might interfere with fetal joint development. Data are limited concerning trimethoprim-sulfamethoxazole, but the drug should probably be avoided during pregnancy, especially the first trimester. Options for treating travelers' diarrhea in pregnant women include azithromycin and third-generation cephalosporins. For symptomatic relief, the combination of kaolin and pectin (Kaopectate; Donnagel) appears to be safe, but loperamide (Imodium) should be used only when necessary. Adequate fluid intake is essential.

Helpful maps are available in the University of Texas Perry-Castaneda Map Collection and the United Nations map library. If you have the name of the town or city you'll be visiting and need to know which state or province it's in, you might find your answer in the Getty Thesaurus of Geographic Names.

Americans living or traveling in Mexico are encouraged to register with the nearest U.S. Embassy or Consulate through the State Department's travel registration website, https://travelregistration.state.gov, and to obtain updated information on travel and security within Mexico. Americans without Internet access may register directly with the nearest U.S. Embassy or Consulate. By registering, you'll make it easier for the Embassy or Consulate to contact you in case of emergency. The U.S. Embassy is located in Mexico City at Paseo de la Reforma 305, Colonia Cuauhtemoc, telephone from the United States: 011-52-55-5080-2000; telephone within Mexico City: 5080-2000; telephone long distance within Mexico 01-55-5080-2000. You may also contact the Embassy by e-mail at: ccs@usembassy.net.mx. The Embassy's Internet address is http://www.usembassy-mexico.gov/.

In addition to the Embassy, there are several United States Consulates and Consular Agencies located throughout Mexico:

Consulates:

Ciudad Juarez: Avenida Lopez Mateos 924-N, telephone (52)(656) 611-3000. Guadalajara: Progreso 175, telephone (52)(333) 825-2998. Monterrey: Avenida Constitucion 411 Poniente, telephone (52)(818) 345-2120. Tijuana: Tapachula 96, telephone (52)(664) 622-7400. Hermosillo: Avenida Monterrey 141, telephone (52)(662) 217-2375. Matamoros: Avenida Primera 2002, telephone (52)(868) 812-4402. Merida: Paseo Montejo 453, telephone (52)(999) 925-5011. Nogales: Calle San Jose, Nogales, Sonora, telephone (52)(631) 313-4820. Nuevo Laredo: Calle Allende 3330, Col. Jardin, telephone (52)(867) 714-0512.

Consular Agencies:

Acapulco: Hotel Continental Emporio, Costera Miguel Aleman 121 - Local 14, telephone (52)(744) 484-0300 or (52)(744) 469-0556. Cabo San Lucas: Blvd. Marina Local C-4, Plaza Nautica, Col. Centro, telephone (52)(624) 143-3566. Cancun: Plaza Caracol Two, Second Level, No. 320-323, Boulevard Kukulcan, km. 8.5, Zona Hotelera, telephone (52)(998) 883-0272. Ciudad Acuna, Ocampo # 305, Col. Centro, telephone (52)(877) 772-8661 Cozumel: Plaza Villa Mar en El Centro, Plaza Principal, (Parque Juarez between Melgar and 5 th Ave.) 2nd floor, Locales #8 and 9, telephone (52)(987) 872-4574. Ixtapa/Zihuatanejo: Hotel Fontan, Blvd. Ixtapa, telephone (52)(755) 553-2100. Mazatlan: Hotel Playa Mazatlan, Playa Gaviotas #202, Zona Dorada, telephone (52)(669) 916-5889. Oaxaca: Macedonio Alcala No. 407, Interior 20, telephone (52)(951) 514-3054 (52)(951) 516-2853. Piedras Negras: Prol. General Cepeda No. 1900, Fraccionamiento Privada Blanca, telephone (52) (867) 788-0343 Puerto Vallarta: Zaragoza #160, Col. Centro, Edif. Vallarta Plaza, Piso 2 Int.18, telephone (52)(322) 222-0069. Reynosa: Calle Monterrey #390, Esq. Sinaloa, Colonia Rodriguez, telephone: (52)(899) 923 - 9331 San Luis Potosi: Edificio Las Terrazas, Avenida Venustiano Carranza 2076-41, Col. Polanco, telephone: (52)(444) 811-7802/7803. San Miguel de Allende: Dr. Hernandez Macias #72, telephone (52)(415) 152-2357 or (52)(415) 152-0068.

Two volcanoes in Mexico have been active in recent years: Popocatepetl, or El Popo, situated 38 miles southeast of Mexico City, and the Volcan de Colima, located on the Jalisco-Colima border. Updated information on these volcanoes may be found at www.cenapred.co.mx and www.ucol.mx/volcan.

For information on safety and security, go to the U.S. Department of State, United Kingdom Foreign and Commonwealth Office, Foreign Affairs Canada, and the Australian Department of Foreign Affairs and Trade.