Dirty water can cause dozens of different illnesses, ranging from a few days of diarrhea to life-threatening infections that damage your kidneys, liver, and lungs. Globally, unsafe water, sanitation, and hygiene were responsible for more than 1.4 million preventable deaths in 2019, according to WHO estimates. Over a million of those deaths were from diarrhea alone. Here’s what’s actually lurking in contaminated water and what each pathogen does to your body.
Bacterial Infections
Bacteria are some of the most common and dangerous organisms found in contaminated water. They typically enter water supplies through sewage overflows, broken sewage systems, polluted stormwater runoff, and agricultural runoff from farms.
Cholera is caused by a bacterium that colonizes the small intestine and triggers the release of massive amounts of water and salt from your gut lining. The result is severe, watery diarrhea that can lead to fatal dehydration within hours. Without treatment, the fatality rate exceeds 50%. With prompt rehydration, it drops below 1%. Cholera is rare in countries with modern water treatment but remains a serious threat in areas with poor sanitation.
E. coli O157 often originates from cattle farms, where it lives harmlessly in the intestines of healthy cows. When manure washes into rivers or reservoirs, the bacteria can reach drinking water. Infection causes bloody diarrhea and stomach cramps, and in some cases leads to kidney failure, particularly in young children. Symptoms typically start within a few days of exposure.
Salmonella causes nausea, vomiting, diarrhea, fever, and cramps that usually appear 6 to 48 hours after exposure, though onset can take up to 10 days. Most people recover without treatment in a few days, but severe cases can become dangerous for young children, older adults, and people with weakened immune systems.
Shigella causes a form of dysentery, with bloody diarrhea, fever, and intense stomach pain appearing 2 to 4 days after exposure. It spreads through water contaminated with infected human waste and is highly contagious, requiring very few organisms to cause illness.
Campylobacter is one of the most frequently detected bacterial contaminants in water. It causes diarrhea (often bloody), cramping, and fever. Most infections resolve on their own, but in rare cases, Campylobacter triggers a nerve condition that causes temporary paralysis.
Parasites That Resist Chlorine
Two parasites stand out because they’re extremely hard to kill with standard water treatment. Cryptosporidium and Giardia form protective shells (called cysts) that allow them to survive even in properly chlorinated water. This is why they’re the leading cause of waterborne illness outbreaks tied to recreational water like pools, water parks, and lakes.
Cryptosporidium symptoms typically begin about a week after swallowing the parasite, though onset can range from two days to four weeks. The main symptom is watery diarrhea that comes and goes for a month or longer. You remain contagious from when symptoms start until at least two weeks after the diarrhea stops, which is why public health officials recommend staying out of pools for two weeks after recovery.
Giardia infects your small intestine and causes greasy, foul-smelling diarrhea, gas, bloating, and nausea. It can persist for weeks if untreated. Giardia cysts are among the most resistant pathogens known. Even UV disinfection, which easily kills bacteria like E. coli at low doses, struggles to fully inactivate Giardia cysts. Boiling water remains the most reliable way to kill both of these parasites.
Viruses in Contaminated Water
Several viruses spread through water contaminated with human fecal matter, typically from sewage overflows or failing septic systems.
Hepatitis A infects the liver and can cause weeks to months of fatigue, nausea, jaundice (yellowing of the skin and eyes), and abdominal pain. Unlike many waterborne illnesses that resolve in days, hepatitis A can leave you feeling sick for months. A vaccine is available and highly effective.
Norovirus is the most common cause of acute gastroenteritis worldwide. It causes sudden, intense vomiting and diarrhea that usually lasts 1 to 3 days. It’s notoriously easy to spread, with just a tiny amount of virus particles enough to cause infection.
Rotavirus primarily affects young children, causing severe diarrhea, vomiting, and fever. Before widespread vaccination, it was one of the leading causes of childhood death from dehydration in low-income countries. Enteroviruses can cause a range of symptoms from mild cold-like illness to more serious conditions including meningitis.
Legionnaires’ Disease From Building Water Systems
Legionella bacteria occur naturally in lakes and streams but become dangerous when they multiply inside building water systems. Unlike other waterborne illnesses, you don’t get Legionnaires’ disease from drinking contaminated water. You get it from breathing in tiny water droplets from showers, cooling towers, hot tubs, or decorative fountains.
The bacteria thrive in warm, stagnant water between 77°F and 113°F (25°C to 45°C), and can grow at temperatures as low as 68°F. Pipes with low water flow, sediment buildup, and insufficient disinfectant create ideal conditions. Legionnaires’ disease causes a severe form of pneumonia with high fever, cough, and shortness of breath. It can be fatal, especially in older adults and smokers.
Leptospirosis From Flood Water
Floodwater carries a risk that many people don’t think about. Leptospirosis is caused by bacteria shed in the urine of infected rodents, livestock, pets, and wild animals. These bacteria can survive in contaminated water or soil for weeks to months. You can become infected by wading in, swimming in, or swallowing floodwater, especially after hurricanes or heavy rains.
Symptoms include fever, muscle aches, headache, vomiting, jaundice, and fatigue. Because these symptoms overlap with so many other conditions, leptospirosis is often misdiagnosed. In 5 to 10% of cases, the infection progresses to multiorgan failure affecting the kidneys, liver, and lungs, which can be fatal.
Chemical Contaminants and Long-Term Damage
Not all waterborne diseases come from germs. Chemical pollutants dissolved in drinking water cause chronic health problems that develop over months or years of exposure.
Arsenic is one of the most widespread chemical contaminants in groundwater worldwide. Long-term exposure causes characteristic skin lesions and significantly increases the risk of skin, bladder, and lung cancer. It’s also linked to cardiovascular disease, diabetes, and heart attacks. In children, arsenic exposure has been shown to impair cognitive development, intelligence, and memory. In pregnant women, exposure is associated with negative developmental outcomes for the baby.
Lead enters water primarily through corroded pipes and plumbing fixtures. It’s especially harmful to children, where it causes irreversible damage to brain development, lowers IQ, and causes behavioral problems. In adults, chronic lead exposure raises blood pressure and damages the kidneys. There is no safe level of lead exposure for children.
How to Make Water Safe
Different contaminants require different treatment methods, and no single approach kills everything. Boiling water for at least one minute is the most universally effective method. It kills bacteria, viruses, and parasites, including chlorine-resistant organisms like Cryptosporidium and Giardia. It does nothing for chemical contaminants like arsenic or lead.
Chlorine disinfection works well against most bacteria and many viruses but struggles with parasitic cysts. Chlorine dioxide is considerably more effective than standard chlorine against Giardia, and ozone treatment is more effective still. UV disinfection easily kills bacteria at standard doses but requires much higher intensity to inactivate Giardia cysts.
If you’re concerned about chemical contaminants, filtration systems rated for the specific chemical are your best option. Reverse osmosis filters remove arsenic, lead, and many other dissolved chemicals. Carbon filters handle some but not all contaminants. No single filter removes everything, so check the certification for the specific pollutant you’re concerned about.