Everyone I have ever been camping with seems to have an unwritten rule never to drink from streams no matter how clear they might seem, for fear of Giardia. I have also heard people telling about having to go to the doctor for a pill and "living" on the toilet for a while.

Even as recent as the 1900s, people have been out exploring and drank from streams with no hesitation, but I don't hear stories about people dying (or suffering greatly) from Giardia.

Basically, my real question is: Have our bodies become weaker somehow? Or why is everyone so scared of it now, when no one seemed to care before?

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    Because it totally sucks and ruins your trip, and it's easy to prevent by using modern filtering. Commented Mar 5, 2016 at 0:33
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    @ChrisMendez: Because it totally sucks and ruins your trip (1) Most Giardia infections produce no symptoms. (2) Evidence shows that you can't get it from backcountry water in the US. (3) It has an incubation period of about a week or two, so even if you did manage to catch it while backpacking, it wouldn't be able to ruin your trip unless it was quite a long trip.
    – user2169
    Commented Mar 5, 2016 at 2:52
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    What changed since the 1900s? A lot more people in backcountry areas pooping all over the place, and a lot more human development and associated pollution in general. The other answers seem to point out with good sources that giardia may not be as big a concern as it's made out to be, there are plenty of other contaminants prevalent in surface water sources which make it wise to treat your water. Given the choice between carrying basic water treatment equipment or spending a lot of time squatting over a cathole, and having done both, I'll take the former every time.
    – nhinkle
    Commented Mar 5, 2016 at 8:54
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    I don't go camping in places where I can't drink the water, I've been drinking out of streams, creeks, rivers, and lakes my whole life and never once been sick.
    – ShemSeger
    Commented Mar 5, 2016 at 15:18
  • 3
    related: outdoors.stackexchange.com/questions/5106/…
    – user2169
    Commented Mar 5, 2016 at 18:14

4 Answers 4


The question doesn't state what geographical area it's about, and it really isn't possible to give an answer that covers everything. In this answer, I'm only going to deal with pristine backcountry areas in North America, such as the Sierra.

In order to interpret the scientific evidence properly, it's necessary to understand some scientific background about Giardia and giardiasis.

The human gut is naturally teeming with microorganisms. These are known as your gut flora or gut microbiome. Most of these are bacteria, but quite a few are other organisms, including protozoans such as Giardia. Some of these critters in your intestines are beneficial or even necessary for your metabolism, while others may be neutral or harmful. People tend to develop tolerance for their own gut flora, but can get sick from other people's. The gut microbiome tends to be more diverse among people in the developing world, less so in the developed world, and this decreased diversity may actually be a bad thing.

Giardia is present in about 3-7% of adults in the US, about 30% in the developing world.[Auerbach 2012] Among toddlers in the US, roughly a third have it.[Ish-Horowicz 1989] Most people who have Giardia as part of their gut flora have no symptoms at all, which is why you don't see a third of toddlers with miserable cases of diarrhea at any given time. Among people who do have symptoms, the condition is normally mild and self-limiting. For unknown reasons, there is a small portion of the population that tolerates Giardia badly if newly introduced to it, and these people have unpleasant diarrhea for some period of time.

Giardia is present in surface water in the form of dormant cysts. These cysts tend to resist being killed by chlorine. You can pick up Giardia by drinking water that contains cysts, but what was not realized in the 1970s, during the initial Giardia public-health panic, was that Giardia is also transmissible through hand-to-mouth contamination. In the context of backpacking, this would something like the following. A and B go backpacking together. A has Giardia in her gut microbiome, and she tolerates it and has never had any symptoms. B doesn't have it. A poops and doesn't wash her hands, and then A and B have dinner together and share pots and pans. B eats food that is contaminated with A's Giardia.

Many people use Giardia as a sort of generic term, like "xerox" or "kleenex." For this reason, it is common to hear backpackers claim that they "got Giardia," when all they really know is that they got sick. They could have had some other condition, they could have had giardiasis but contracted it somewhere else, or they could have gotten a bug through hand-to-mouth contamination from their hiking partners.

Often people will make this claim when they got sick during the hike. This is unlikely. When an animal is infected by a parasite, there's a prepatent period, which is the time from infection with a parasite to when the bugs reach a life stage where they can be detected by a lab test. There's also an incubation period, which is the time from infection to symptoms. For most parasites, the prepatent period is shorter than the incubation period, but for Giardia it's often longer. A 1954 study on prison volunteers showed an average prepatent period of 9 days, but there's a wide range of variation, and the incubation period can be as long as months. In a study of travelers to the Soviet Union, the typical time until acute symptoms occurred was found to be a couple of weeks. In about two thirds of patients, the prepatency period was longer than the incubation period by a week or more. In summary, if someone gets backpacker's diarrhea while on a weekend backpacking trip, it's very unlikely that it was caused by giardiasis that they acquired during the trip.

As stated above, my answer only deals with pristine backcountry areas in North America. Some studies have surveyed water in these areas for Giardia.[Suk 1986],[Jaret 2003] The water was extremely clean, and huge volumes of it had to be filtered in order to pick up any detectable number of Giardia cysts. For example, there were sites in the Sierra where they filtered 100 gallons of water and didn't detect a single Giardia cyst left over in the filter. In most of the locations where cysts were detected, the concentrations were so low that they have to be expressed in scientific notation. In low-use areas, they ranged from zero to about 5x10^-3 per liter, while one high-use area had about 0.1 per liter. Elsewhere in the U.S., similar testing also found extremely low concentrations all of the backcountry locations tested: West Beaver Creek, AZ; Merced River, CA; Chattooga River, NC; Neversink River, NY; White Pine Lake, UT; Greenwater River, WA; and Renard Lake, WI.

From these studies, it appears that if you spend a weekend drinking untreated water in pristine backcountry areas in the US, you will typically not ingest a single Giardia cyst. We then need to do a risk-benefit analysis. This raises the question of how many cysts you need to swallow in order to have a certain chance of getting infected, as well as the chance that this will cause symptoms. The best single source of information on the first question is a 1954 study by Rendtorff that used prison volunteers; the data are summarized and analyzed further by [Cox 2002] and [Rose 1991]. Roughly speaking, you have to ingest about 20-30 cysts to be likely to get an infection.

It would be interesting to know whether there is a threshold effect, i.e., whether or not there is some chance, however small, of getting sick by swallowing a single cyst. The data are not sufficient to determine this. Rose introduces a mathematical model in which it is assumed that each cyst has some probability p of setting up shop in your gut, but this is an assumption of the model, and is not testable based on the data, which used higher doses. Rose's model is also not consistent with data showing that even when people drink very highly contaminated water, there is still only about a 50% chance of contracting giardiasis.[Wilkerson 1992] In this type of study, one of the confounding factors is that the minimum infectious dose can vary depending on the strain of the microorganism.

If you do pick up a giardia infection, and if you weren't already an asymptomatic carrier, then it appears that your chance of developing symptoms is about one in 10.[Wilkerson 1992]

Based on these numbers, we can make at least a rough order-of-magnitude estimate of the risk associated with drinking backcountry water. Suppose you go on a weekend hiking trip in the Sierra and drink 6 liters of untreated water. In low-use areas, the concentration of Giardia cysts in your water appears to average about 3x10^-3 per liter. Based on Rose's model, take the probability of infection to be about p=.02 per cyst. If infection occurs, the chance of getting symptoms (which are in most cases mild) is on the order of 0.1. Multiplying these factors, we arrive at a probability of about 4x10^-5 that you will get Giardia symptoms. That is, under these assumptions, out of a million people who do this, about 40 are expected to get diarrhea.

This estimate appears to conflict with a study by Zell,[Zell 1993] which states:

The incidence of Giardia cyst acquisition in backcountry travelers was only 5.7% (95% CI 0.17–20.2%). Mild, self-limiting gastrointestinal illness occurred in 16.7% of subjects (95% CI 4.9%–34.50%), none of whom demonstrated G. lamblia infection.

Although none of the people in the study who had Giardia got symptoms, this rate of infection is many orders of magnitude higher than would have been expected from water contamination based on the Rendroff-Rose data and modeling. Unfortunately the Zell article is paywalled, so I can only see the abstract, but it appears that he would have had no way to tell whether the people who acquired infections got them from contaminated water or from hand-to-mouth contamination. In any case, Zell's conclusion is that water treatment is of marginal cost-effectiveness, given the low risk and the fact that the infection is self-limiting and usually asymptomatic.

Another reality check we can do is to compare the concentrations of Giardia in backcountry water with the concentrations in city tap water. This is complicated by the fact that Giardia cysts, whether found in backcountry water or tap water, may be nonviable, and surveys cannot usually determine their viability. Historically, city tap water has been getting cleaner and cleaner, especially in the developed world. However, it appears to have been normal in 20th-century America for city tap water to contain concentrations of Giardia cysts that are similar to or greater than the concentrations found in backcountry areas of the US.[Rockwell 2002] It would be interesting to get more detailed information on this from someone who has professional-level knowledge of US water supplies and their history.

A meta-analysis of the literature in 2000 concluded that "the evidence for an association between drinking backcountry water and acquiring giardiasis is minimal."[Welch 2000] When people do actually contract backpacker's diarrhea from exposure during a hiking trip, by far the most common reason is hand-to-mouth contamination.[Welch 1995]

Auerbach 2012 - Paul S. Auerbach, Wilderness Medicine (6th ed., 2012), ch. 68

Cox 2002 - Cox, F.E.G. (2002). History of Human Parasitology. Clin. Microbiol. Rev. 15(4): 595

Erlandsen 1984 - Erlandsen, Giardia and giardiasis: biology, pathogenesis, and epidemiology, 1984.

Ish-Horowicz 1989 - Ish-Horowicz et al., "Asymptomatic giardiasis in children," Pediatr Infect Dis J. 1989 Nov;8(11):773-9.

Jaret 2003 - Peter Jaret, "What's In the Water?," Backpacker, Dec. 2003, p. 45.

Jokipii, The Lancet, Volume 309:1095.

Rockwell 2002 - Robert L. Rockwell, Sierra Nature Notes, Volume 2, January 2002, http://web.archive.org/web/20051026030831/www.yosemite.org/naturenotes/Giardia.htm

Rose 1991 - Rose, Haas, and Regli, "Risk assessment and control of waterborne Giardiasis," Am J Public Health 81 (1991) 709, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1405147/pdf/amjph00206-0039.pdf

Suk 1986 - Map cited as reference 17 in S.C. Zell, "Epidemiology of wilderness-acquired diarrhea: implications for prevention and treatment," Wilderness and Environmental Medicine 3 (1992) 241, http://www.wemjournal.org/article/S0953-9859(92)71235-2/abstract

Welch 1995 - Thomas R. Welch and Timothy P. Welch, "Giardiasis as a threat to backpackers in the United States: a survey of state health departments," Wilderness and Environmental Medicine, 6 (1995) 162, http://www.wemjournal.org/article/S1080-6032%2895%2971046-8/abstract

Welch 2000 - Welch, T.P. "Risk of giardiasis from consumption of wilderness water in North America: a systematic review of epidemiologic data," Int J Infect Dis. 2000;4:103100, http://download.journals.elsevierhealth.com/pdfs/journals/1201-9712/PIIS1201971200901024.pdf?refuid=S1080-6032(04)70498-6&refissn=1080-6032&mis=.pdf

Wilkerson 1992 - Wilkerson, James A., MD: Medicine for Mountaineering and Other Wilderness Activities. The Mountaineers, 4th edition, 1992 (referenced in Rockwell, http://web.archive.org/web/20051026030831/www.yosemite.org/naturenotes/Giardia.htm )

Zell 1993 - Zell and Sorenson, "Cyst acquisition rate for Giardia lamblia in backcountry travelers to Desolation Wilderness, Lake Tahoe," Journal of Wilderness Medicine 4 (1993) 147.

  • 1
    I disagree, having read other papers (there's research on both sides of this), but still giving you the upvote for doing all the work of putting this together. Your conclusion is a viable one. Commented Mar 7, 2016 at 13:19
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    I also disagree. Doctors Welch and Rockwell have been refuted by the CDC, and their conclusions do not reflect the current scientific consensus. Please see my below post. Commented Mar 11, 2016 at 1:38
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    I have asked the question at Skeptics.
    – gerrit
    Commented Mar 11, 2016 at 14:53
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    @gerrit - Thanks for letting me know about the cross-post. I've had many negative experiences in the past with skeptics.SE, but decided to try again with them, so I rewrote this answer for them, with much more detail, more references, and less dependence on the Rockwell article and more on the primary scientific literature. I posted the rewrite there and also replaced my answer here with the new version. The response on skeptics.SE was in keeping with my past negative experiences, so I deleted the answer on that site. But anyway I'm glad to have been motivated to improve the one here.
    – user2169
    Commented Mar 14, 2016 at 5:04
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    Looks like we're not getting any further under the watchful eye of the Skeptics regulars.
    – gerrit
    Commented Mar 14, 2016 at 10:27

I'm going to chime in on a more practical answer. The question is not "What is the scientific case for Giardia". The question is "Why are people so scared of it?" "Or why is everyone so scared of it now, when no one seemed to care before?" Or in other words, we're not talking about how dangerous Giardia is, but rather why people are worried; specifically in relation to the "unwritten rule to never drink from streams"

It is a fact that biologically contaminated water exists and can make you sick. And to the "the water is safe" crowd - I'm sorry you are just wrong. Surface water is quite often biologically contaminated. There are millions of people in the world who would love to have one of the filters us hikers carry all the time. Yes you can get sick via the fecal oral route. That does not mean water is safe. Don't believe me? Take a trip to a random country and drink out of every stream you find... you'll change your mind.

So why Giardia specifically?

Giardia has become, in hiker parlance, a rather generic term. People use it to cover pretty much any sort of illness-due-to-bad-water (Gastrointestinal infection). Even doctors. In my own case I had Amoebic Dysentery but I would not have known that had I not read my lab reports. My doctor just called it "Giardia" (they did prescribe the right medicine).

Why be scared of diarrhea caused by bad water? Unless, like Ben Crowell, you have steel bowels then it just sucks. Imagine having uncontrollably explosive diarrhea for weeks to the point that you can't even drive to work because there aren't enough bathrooms along the way. Then imagine having intestinal problems for months after that due to how much it's screwed with your system. (And then imagine people assuring you on forums that back country water is safe and you're just over-reacting) That's what Amoebic Dysentery is like, at least for some. I know. I had it.

Giardia, in common usage, is just a synonym for what non-hikers call travelers diarrhea. And that is why people are scared of it. When they hear "Giardia" they are not thinking of one illness, but all the possible illnesses in an entire category called "Gastrointestinal infections". Historically these all fell under a malady call the "bloody flux." If you want to find historical cases of death, look there. Remember that it is only very recently that we could test to distinguish between the causes.

But no one is going to say "Gastrointestinal infection" and we don't call it "the bloody flux" anymore. Lay people are just not all going to use (or care about) that level of diagnostic detail. They're going to call it Giardia in the same way we call a copy machine a Xerox machine and there's absolutely zero point (from a practical standpoint) in arguing over the difference when the prevention is the same:

Filter/Boil your friggin water.

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    Good point about how "Giardia" is often used as a generic term. I took the question to be solely about Giardia, and my answer addressed only Giardia. But reading the question, you're right, it could be taken much more broadly, like "Xerox." And to the "the water is safe" crowd - I'm sorry you are just wrong. I guess this is supposed to include me? I've never said that all water is safe. The very first sentence of my answer notes that the answer depends on where you are. My answer also gives a lot of geographical specifics, including lists of places that have been studied.
    – user2169
    Commented Mar 8, 2016 at 1:10
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    There is a problem with the approach of lumping together all pathogens and lumping together all water treatment methods. The problem is that different treatment methods work against different pathogens. For example, you say, "Filter/Boil your friggin water." Filtering doesn't work against viruses. Boiling kills every disease-killing organism, but boiling is time-consuming and the reality is that people often aren't going to take that much time. I don't think it's helpful to oversimplify the science. If you do that, then you're just encouraging people to go through a voodoo ritual.
    – user2169
    Commented Mar 8, 2016 at 1:14
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    @BenCrowell -- "The water is safe crowd" was not to include you, no :). I did joke a bit at your steel intestines because (from my view) you seemed to suffer much less than me from that sort of infection. Very salient point on the viruses. Though while filtering may not prevent viruses, drinking it straight with no filter stops nothing at all. It's hardly a voodoo ritual just because it's not 100% effective. To call it that would be a bit like calling condoms a voodoo ritual because they don't prevent HIV 100% of the time. Commented Mar 8, 2016 at 13:29
  • OK, that's reasonable. +1.
    – user2169
    Commented Mar 14, 2016 at 4:58

Why are People so Worried About Giardia?

Because giardia is commonly found in backcountry water and it is known to make some people very sick. The EPA says [giardia] "Cysts have been found all months of the year in surface waters from the Arctic to the tropics in even the most pristine of surface waters." [a]

Or why is everyone so scared of it now, when no one seemed to care before?

Its role as a pathogenic organism was not recognized until the 1970s Additionally, while most wilderness-acquired diarrhea normally resolves itself in a day or two, symptomatic giardiasis is often much more serious. One study of verified cases found that the average victim of giardiasis was sick 3.8 weeks and lost 12 pounds. [d] In rarer cases giardiasis may lead to more serious complications. [L]

The paper "Risk assessment and control of waterborne giardiasis" found that the minimum infective dose is ONE giardia cyst with one cyst estimated to be a 2% risk. [b] [i] It is a fallacy that it takes a minimum of 10 cysts for a giardiasis infection to occur. That is based on a misrepresentation of the famous Rendtorff study. [h]

There have been documented outbreaks among backpackers from drinking water, as reported in "Outbreak of Giardiasis Associated with Drinking Surface Water Along a Hiking Trail" (2015) and "An outbreak of giardiasis in a group of campers" and others.[c] [f] Less than 1% of giardiasis cases are part of outbreaks.[g]

The paper "Giardiasis in Colorado: an epidemiologic study" which looked at only lab-verified giardiasis cases concluded: an increased incidence of giardiasis [was shown for those who] camped out overnight (38% vs. 18%), and drank untreated mountain water (50% vs. 17%), p less than .001. Also identified was a correlation between the seasonal distribution of cases and degree of fecal contamination of mountain streams….drinking untreated mountain water is an important cause of endemic infection [my bold] [d]

As of 2016, there is a broad consensus by the CDC, FDA, EPA, Mayo Clinic and state health departments that giardiasis is common [with an estimated 1.2 million cases annually, with about half or less symptomatic] and that campers and hikers drinking untreated surface water are among those at greatest risk [a] [e] [i] [j]

I think the CDC summarized the issue well: Although the advice to universally filter and disinfect backcountry drinking water to prevent disease has been debated, the health consequences of ignoring that standard water treatment advice have been documented… [k]

I wrote this article about what science says about treating backcountry water. It contains numerous citations.

I put together this long list of scientific papers related to backcountry water pathogens.

I don't worry about treating water, I just do it. It's easy, a lot easier than dealing with three bad cases of giardiasis, I can tell you that from personal experience.


a: Environmental Protection Agency “Giardia: Drinking Water Health Advisory”

b: Rose, Joan B., Charles N. Haas, and Stig Regli. “Risk assessment and control of waterborne giardiasis.” American journal of public health 81.6 (1991): 709-713.

c: Gretsch, Stephanie. “Outbreak of Giardiasis Associated with Drinking Surface Water Along a Hiking Trail.” 2015 CSTE Annual Conference. Cste, 2015.

d: WRIGHT, RICHARD A., et al. “Giardiasis in Colorado: an epidemiologic study.” American journal of epidemiology 105.4 (1977): 330-336.

e: Painter, Julia E., et al. “Giardiasis Surveillance—United States, 2011–2012.” Morbidity and mortality weekly report. Surveillance summaries (Washington, DC: 2002) 64 (2015): 15-25.

f: Barbour, Alan G., Craig R. Nichols, and Taira Fukushima. “An outbreak of giardiasis in a group of campers.” The American journal of tropical medicine and hygiene 25.3 (1976): 384-389.

g: Petersen, Lyle R., Matthew L. Cartter, and James L. Hadler. “A food-borne outbreak of Giardia lamblia.” The Journal of infectious diseases (1988): 846-848.

h: RENDTORFF, ROBERT C. "The experimental transmission of human intestinal protozoan parasites. II. Giardia lamblia cysts given in capsules." American Journal of Hygiene 59.2 (1954): 209-20.

i: Microorganisms, Foodborne Pathogenic, and Natural Toxins Handbook. "Bad Bug Book." Center for Food Safety & Applied Nutrition. Accessed October 24 (2005).

j: Mayo Clinic, Diseases and Conditions Giardia infection (giardiasis)

k: Yoder, Jonathan, et al. “Surveillance for waterborne disease and outbreaks associated with drinking water and water not intended for drinking–United States, 2005-2006.” Morbidity and mortality weekly report. Surveillance summaries (Washington, DC: 2002) 57.9 (2008): 39-62.

l: Halliez, M. C., and André G. Buret. "Extra-intestinal and long term consequences of Giardia duodenalis infections." World J Gastroenterol 19.47 (2013): 8974-8985.

  • Comments are not for extended discussion; this conversation has been moved to chat.
    – Rory Alsop
    Commented Mar 8, 2016 at 7:56
  • I have asked the question at Skeptics.
    – gerrit
    Commented Mar 11, 2016 at 14:53
  • 2
    @gerrit -- Skeptics isn't a very good resource IMO. Science is not best resolved by popular vote. Commented Mar 13, 2016 at 18:05

Some excellent research-based answers here, so I'll focus on the practical implications.

First, and most importantly:

Most intestinal infections in the backcountry aren't caused by bad water - the main danger is poor personal hygiene in a setting where many allow standards to slip.

So be scrupulous yourself, and be wary of sharing food that has been handled by others.

As for waterborne giardia (and other infectants that are generically bundled under this name) I just don't see the point in taking any risks - I always carry a lightweight filter. It weighs 100 grams and works down to the viral level. If there's the slightest doubt I use it. It's very quick and low hassle, while the infections it prevents are very nasty. So the cost/benefit calculation is a no-brainer.

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Not everywhere is as pristine as the High Sierra. In the montane zone where you are mixing with livestock a filter is pretty much essential as fecal contamination is a real risk. And even higher up many wild animals carry giardia and other infections.

I bought my filter after a traverse of the Spanish Sierra Nevada in an unseasonal autumn heatwave. There was water all over the map so I didn't anticipate any issues. But there were herds of Ibex peeing and pooing into every source as they stood in the water to cool down. I had a real struggle finding anything safe. Dehydration in a heatwave is no joke...

(I also carry my filter in towns - I can drink from any source and it saves me a fortune in ecologically damaging bottled water).

So the answer is that if you are careful about hygiene and carry a good filter there is no reason to be concerned about giardia. Without these precautions, in many situations there is reason to worry as others have shown.

  • 3
    I'm with you on this. It's so easy and simple to prevent many of the risks, so why not do so? Commented Mar 8, 2016 at 13:30
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    I do understand how simple it is, but why didn't anyone worry about it before? It's not like animals didn't have to pee back before the 1950s. Commented Mar 8, 2016 at 19:06
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    @JustWondering -- The point of my post is that people did worry about it before. You have to remember that medicine and diagnostic procedures have changed in the past 60 years. People worried about it very much they just didn't necessarily know what caused it, how to prevent it, called it something different, etc. Clean water has most certainly been an issue since before 1950. Commented Mar 8, 2016 at 19:28
  • @Russell Steen -- Thank you for the clarification. Commented Mar 8, 2016 at 20:02
  • 1
    @JustWondering In the 1960s no-one walking in the Scottish hills worried about water. On one trip 4 of us drank from an infected stream, and the effects were very unpleasant indeed. The issue was there, we just didn't have the quality of filter that would have allowed us to deal with it. Commented Feb 27, 2017 at 0:40

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