Most illness that travelers experience in Mexico is gastrointestinal, caused by bacteria their immune systems have never encountered. The pooled incidence rate for traveler’s diarrhea in Latin America and the Caribbean is roughly 39 cases per 100 person-months of travel, making it one of the highest-risk regions in the world. The good news: these illnesses are usually short-lived, with a median symptom duration of about three days without treatment and closer to one and a half days after treatment.
The Bacteria Behind “Montezuma’s Revenge”
The primary culprit is a specific strain of E. coli called enterotoxigenic E. coli, or ETEC. This bacterium produces toxins that trigger your intestines to release fluid, resulting in watery diarrhea. It’s considered the single most common cause of traveler’s diarrhea worldwide, and it’s especially prevalent in Mexican food and water sources. A study of U.S. travelers to Guadalajara found ETEC in a significant share of diarrhea cases, with most strains producing a heat-stable toxin that resists cooking temperatures.
ETEC isn’t the only bacterial threat. A related strain called enteroaggregative E. coli (EAEC) causes a different pattern of illness that can linger longer, sometimes leading to persistent diarrhea lasting a week or more. Research on foods obtained from Mexican homes and commercial sources has also recovered Salmonella, Shigella, and other illness-causing bacteria. Salmonella was the most frequently found pathogen after E. coli, appearing in 17 isolates from home-prepared foods in one study.
The key point: your body has no built-in defense against these particular bacterial strains. People who live in Mexico develop a degree of immunity through repeated low-level exposure over years. Your gut microbiome simply hasn’t seen these organisms before, so even a small dose can cause symptoms.
Why the Water Is Different
Mexico operates over 2,500 municipal wastewater treatment plants, but they only process about 64% of the wastewater collected through sewage systems. The remaining third enters the environment partially treated or untreated. This gap means that bacteria and parasites can cycle back into water supplies, particularly in rural areas and smaller cities where infrastructure is older or underfunded.
Even in areas where water receives chlorination, the distribution system itself can introduce contamination. Aging pipes, intermittent water pressure, and cross-connections between sewage and water lines allow bacteria to enter tap water between the treatment plant and your faucet. This is why locals drink purified or bottled water too. It’s not that their bodies can handle contaminated water indefinitely; it’s that they’ve adapted to the specific low-level bacterial exposure while also relying on purified water for drinking.
Food Is the Bigger Risk
Many travelers assume water is the main danger, but contaminated food accounts for a large share of illness. Street food, fresh produce washed in untreated water, and meals prepared in home kitchens all carry risk. One laboratory analysis found that foods from Mexican homes commonly contained E. coli and occasionally the toxin-producing strains responsible for traveler’s diarrhea. Salmonella and Shigella appeared exclusively in home-prepared foods, not in commercially sourced ones.
Fresh produce is a particular concern. Cilantro imported from Mexico has been linked to outbreaks of cyclosporiasis, an intestinal illness caused by a parasite, for more than three decades. In one 2023 outbreak at a restaurant in Alabama, eating cilantro was associated with a 41-fold increase in the odds of getting sick. The parasite responsible, Cyclospora, is difficult to wash off produce and isn’t killed by standard rinsing.
Fruits and vegetables that grow close to the ground or are eaten raw pose the highest risk. Anything you can peel yourself (bananas, oranges, mangoes) is generally safer than leafy greens, herbs, or pre-cut fruit from a buffet or street vendor.
What About Resorts and Hotels?
Major resorts and all-inclusive hotels in Mexico have largely solved the water problem with on-site filtration and purification systems. Ice at reputable resorts is made from filtered water, and drinking water served at restaurants within the property goes through the same treatment. This is a significant change from decades past, when ice was a common source of illness for tourists.
That said, resort guests still get sick. The reasons are usually food-related: buffets where dishes sit at lukewarm temperatures, salsas and garnishes made with fresh herbs, or excursions off-property where you eat at local restaurants without the same filtration infrastructure. Legionellosis outbreaks have also been reported at resorts in Mexico, linked to contaminated water in hot tubs, decorative fountains, or air conditioning systems rather than drinking water.
Less Common but Serious Illnesses
Traveler’s diarrhea gets the most attention because it’s the most frequent problem, but Mexico carries risk for several other infections. Hepatitis A is endemic throughout the country, spreading through contaminated food and water. The CDC recommends vaccination for all travelers regardless of where in Mexico you’re headed. Typhoid fever is a concern in smaller cities and rural areas with limited sanitation, and vaccination is recommended for those itineraries.
In 2023, an outbreak of Rocky Mountain spotted fever in Baja California caused multiple deaths. This tick-borne illness isn’t related to food or water but highlights the importance of insect protection in certain regions. Measles and mumps outbreaks have also occurred in Mexico, coinciding with broader global surges, making up-to-date routine vaccinations important before travel.
How to Lower Your Risk
The single most effective precaution is choosing your food and water carefully. Stick to bottled or purified water for drinking, and avoid raw vegetables, unpeeled fruits, and fresh herbs at places you can’t verify use filtered water. Cooked food served hot is far safer than room-temperature buffet items or raw preparations.
For travelers who want an extra layer of protection, bismuth subsalicylate (the active ingredient in Pepto-Bismol) taken as two tablets four times daily has been shown to reduce the risk of traveler’s diarrhea by about 65%. This regimen appears safe for up to three weeks of use. A lower dose provides roughly 40% protection. It won’t eliminate the risk entirely, but it meaningfully lowers the odds.
If you do get sick, most cases resolve within three days without any medical intervention. People who seek treatment typically feel better within a day and a half. Staying hydrated is the most important thing you can do during an episode. Oral rehydration solutions, available at any Mexican pharmacy, replace both fluids and electrolytes more effectively than water alone.