As I mention in another thread, my wife and I are thinking of visiting Playa Del Carmen in Mexico for a week next year. I have visited Mexico in the past, and there is 1 in 4 chance I will get sick as a result of ingesting tap water inadvertently, for example in a shower or eating washed fruits or vegetables.
Is there a drug/medicine I can take as a prophylactic to avoid getting sick if I do happen to ingest some water accidentally?
A med as a preventative? No. A med to take at the first sign of symptoms? Perhaps.
Since the causative agent is usually enterotoxigenic Escherichia coli, a quinolone antibiotic can help resolve the infection quicker, if taken early. Some docs who specialize in travel medicine will provide a script for same, only to be used IF symptoms appear.
Taking the med prophylactically would tend to select for acquiring a diarrhea resistant to antibiotics, so should be avoided.
If you get the squirts, the best first treatment is to keep hydrated. And this is usually the only treatment needed.
Is there a simple way to avoid ingesting this bacteria in the first place? Tap water can be hard to avoid 100% of the time.
Wear a bag over your head? 
Seriously, there are billions of bacteria everywhere, and you will get incidental exposures, in the shower, in the pool, in the ocean. Do your best to drink only bottled liquids and keep your mouth shut the rest of the time, and if you can’t do that, don’t swallow! 
Just the usual travel advice: eat only cooked food (no raw fruits, veggies, juices), no ice in your drinks, drink bottled water/soda/beer, use bottled water for brushing your teeth, avoid swallowing water in the shower, wash your hands with soap frequently.
There’s no way to separate ingesting water from ingesting bacteria/viruses.
I’d also like to re-emphasize QtM’s point about not taking the antibiotics as a preventative. This puts you at risk of infections much worse than the typical traveler’s diarrhea. Don’t do it.
Every hotel, restaurant, roadside stand, etc. will serve bottled water and ice made from purified water, and will wash their produce in purified water. It is not very common to get the Montezuma’s revenge from water in PDC but obviously don’t drink tap water and don’t swallow water in the shower if you can avoid it.
Much more dangerous, but delicious nonetheless, are the many roadside treats, tacos, sandwiches, etc. you can get along most every street. The bout of runs you get probably won’t be from water but from someone not washing their hands before preparing the food, or having it sit in the sun a little too long.
Most often the traveller’s blues down here are caused by too much drinking (of alcohol that is), not enough sleep, hot and humid weather and lots of activity in the sun. Stay cool and hydrated, get rest in the shade, and you should be fine.
Can this be elaborated upon? Why does taking an antibiotic as a preventative measure make you more susceptible? I’d figure that susceptibility to a bacteria is a function of exposure and is independent of taking antibiotics.
Depending on where you’re staying, they may guarantee potable water. Most civil systems in Mexico deliver potable water to the premises these days (although improper cistern care can reintroduce microbes to the water supply, but that’s the property owners’ issue).
If you eat where others eat, your chances of having a problem are close to nil. For some reason, though, salsa asada and I seem to have issues. The only times I’ve ever been sick living in Mexico was after eating salsa asada in different restaurants. (Yes, I still eat it.)
The website seems to be not responding right now, but Quinta Roo Salubridad should have restaurant health requirements and sample charts if you have concerns.
After drinking the water there and getting sick a few times, wont a person get an immunity to them just like the locals?
Here in the Yucatan, the “locals” don’t drink the tap water. It may be the taste. They use a lot of chlorine. Or, it maybe due to the water storage before it comes out of the tap. As was mentioned up thread.
When it comes to water, everyone drinks and cooks with purified water.
Not every single pathogen that enters your body will succeed in infecting you - in many cases, and especially with small doses, your immune system will overwhelm it immediately, or it will just die off because of competition from other organisms.
Antibiotics tend to kill off your body’s own flora - which are normally competing for resources with any foreign pathogen. Without (or with diminished) competition, an antibiotic-resistant pathogen has a greater chance of prevailing.
This. More importantly, any population of bacteria will have some members who are naturally resistant to an antibiotic. Prophylactic overuse of antibiotics kills all but those bacteria and with each generation, the percentage of resistant bacteria increases.
So, taking an antibiotic when you are not ill can guarantee that when you do get ill, the antibiotic will not help.
If they use a lot of Chlorine doesn’t that kill the bad bacteria? I assumed the tap water issues in Mexico were related to their water not being treated with chemicals. What am I missing here?
On my various trips to South and Central America, I’ve often brought along Cipro (by prescription) as a prophylactic measure. High doses for short periods of time. The locals have often been appalled that I’d eat their local flora raw (having seen how it affects gringos), but I’ve never had a problem. That said, drugs like Cipro can have adverse effects if taken for extended periods of time.
Brush with bottled water and keep your mouth closed in the shower.
On street food: It is indeed very yummy and I’ve never had a problem with it. Just go for the stalls with high heat and high turnover and don’t eat raw veggies. Restaurant food can sit around for hours under heat lamps and that’s more likely to make you sick.
Due to an unpredictable supply and/or unannounced rationing, water is often kept by individual users in underground cisterns and then pumped to rooftop storage tanks to let gravity provide internal pressure via water column. The cisterns are easily contaminated with above-ground debris, rainwater, seepage, insects, etc.
Where I live, groundwater from an aquifer is drawn to a municipal treatment plant and deposited through city plumbing in an underground storage tank (enclosed PVC system encased in a cement cell) with a submerged pump and a separate pneumatic to pressurize the plumbing. We only use an activated carbon filter to remove hardness.
Water pressure delivered from the street to residences, businesses, etc. varies a great deal. Depending on distance and diameter of the pipe. At times it may be zero.
For this reason, tinacos are used.
http://www.clinchtrails.com/posts/Tinaco:_Mexican_raised_water_tank/
They are huge (900 liters or more) plastic tanks, positioned on the roof. This provides pressure for the taps below, and guarantees a constant flow of water, due to the storage.
The problem arises when these tinacos aren’t maintained. It is recommended that they be cleaned every six months. This doesn’t happen. I have seen tinacos without a lid. It became a bird bath. Delivering the bird bath water to the faucets below.
While the street water delivered at the street maybe within bacterial standards, there are complications after the delivery.
Well you can filter it (using something like this: http://www.rei.com/product/695265/msr-miniworks-ex-water-filter), boil it, zap it with UV (http://www.rei.com/product/847549/steripen-ultra-water-purifier), or add iodine
Brian
There are many ways to be sick while traveling.
There is often an adjustment period to a new flora and new habits (eating, drinking, sleeping and spending the day differently). This can happen in London as easily as if can happen in Accra. But in London you are more likely to blame the extra Gin and Tonic than the water.
Then there is genuine food poisoning. I have seen a couple hundred people live for years at a time in countries without safe drinking water. There is a clear pattern.
People who eat and drink freely, under the attitude “when in Rome”, spend much of their time sick. People who mostly follow the rules but occasionally let one slide get sick from time to time (this was me.) The people who are fastidious about their water don’t get sick, or get sick rarely.
Of course, it also involves trade-offs. On a short trip, it provably makes sense to be more cautious. But if you are spending years someplace, you may decide that it’s worth the occasional bad night to be able to eat the street food.
Most urban-ish areas have some kind of water treatment. The question is how well staffed, supplied and maintained it is. Parts can be expensive and hard to source, and supplies are not always delivered on schedule.
A water pump once broke in my local “big city”,’ which left the more elevated parts of the city suddenly without water pressure. You’d leave the taps on before you went to bed, and hope that enough water trickled into the buckets overnight to get through the next day.
It stayed that way for several years. Once something breaks, it’s totally possible that it just won’t get fixed. So you can’t rely on municipal water systems.
The locals? They get sick, too. Diarrhea is still the number one killer of children. Productivity in developing countries sucks, in part because people spend much of their working lives sick. As a relatively wealthy foreigner, you don’t have to worry about the costs of wood to boil fire or chlorine to purify it, but many people don’t have ready access to even basic purification methods.
Beyond following the rules to the letter (and definitely don’t get lax just because you are at a tourist restaurant- those are often the worst because they don’t rely on repeat business), there isn’t a lot that you can do. Some doctors will prescribe Cipro for travelers, and that can stop something bacterial in its tracks, but of course won’t stop something viral. And Cipro can have some intense side-effects (I personally will never take it again.)
The problem isn’t drinking water- there is hardly a corner of the earth that doesn’t have cheap bottled water. The problem is fruit that has been washed in bad water, the droplets of water on your freshly washed plate, the water in your melting ice cube, etc.
There is bad water in places you’d never imagine. You’d think a slice of freshly cut watermelon would be as safe as it gets, with that thick rind. Except that vendors will inject water into watermelons to make it juicier.