A waterborne disease is any illness caused by bacteria, viruses, or parasites that spread through water. You can get infected by drinking contaminated water, but also by breathing in water droplets, or simply getting contaminated water in your ears or nose. Globally, about 2.2 billion people lack safely managed drinking water, and diarrheal illness from contaminated water and food kills roughly 444,000 children under five every year.
How Waterborne Pathogens Reach You
The classic route is swallowing water contaminated with fecal matter, often called the fecal-oral route. Sewage leaks into a river, rain washes animal waste into a well, or aging pipes let bacteria seep into tap water. But ingestion isn’t the only path. Legionella, the bacterium behind Legionnaires’ disease, spreads when people inhale tiny water droplets from cooling towers, showerheads, or hot tubs. Parasites like Cryptosporidium can enter your body if you accidentally swallow pool or lake water while swimming.
Weather plays a direct role in how pathogens move. Heavy rainfall saturates soil and flushes germs from the surface into drinking water sources, especially in areas with limited treatment infrastructure. In the U.S. and Canada, waterborne disease outbreaks frequently follow extreme rainfall events. Droughts create the opposite problem: lower water flow concentrates pathogens in rivers and reservoirs, raising infection risk even as water volume shrinks.
The Main Pathogens Behind Outbreaks
Waterborne germs fall into three broad categories: bacteria, viruses, and parasites. In the United States, the CDC tracks outbreaks tied to drinking water and breaks them into two main groups based on how the pathogen behaves in plumbing systems.
The first group, biofilm-associated pathogens, lives in the slimy buildup inside pipes and water systems. Legionella dominates this category, accounting for 86% of all drinking water outbreaks reported to the CDC between 2015 and 2020. It thrives in warm, stagnant water and is especially dangerous in large buildings with complex plumbing.
The second group is infectious enteric (gut) pathogens. Among outbreaks in that five-year window, three organisms caused 94% of the illnesses: norovirus, Shigella, and Campylobacter. Other enteric pathogens tracked in outbreaks include Cryptosporidium, Giardia, and E. coli. Cryptosporidium is considered the leading cause of waterborne disease in the U.S. overall, partly because it resists chlorine disinfection at standard levels.
What Waterborne Illness Feels Like
Most waterborne infections hit the gut. Watery diarrhea, stomach cramps, nausea, and vomiting are the hallmarks, though severity ranges from mild inconvenience to life-threatening dehydration depending on the pathogen and the person’s immune health.
Cryptosporidiosis typically starts about seven days after exposure. The defining symptom is prolonged, frequent, watery diarrhea that can come and go for up to 30 days. Most healthy people recover fully within two to three weeks, but for people with weakened immune systems (from HIV, cancer treatment, or organ transplants) the infection can spread beyond the intestines to the respiratory tract or biliary system and become severe.
Giardia has a similar timeline, with symptoms appearing about seven days after exposure: diarrhea, gas, cramps, nausea, and persistent fatigue. E. coli infections (specifically the hemorrhagic type) develop faster, usually within three to four days, and often cause bloody diarrhea with severe abdominal cramps but little or no fever. Cholera, caused by the bacterium Vibrio cholerae, produces sudden watery diarrhea that can lead to dangerous dehydration within hours if untreated. It remains a major threat in regions without reliable water treatment.
Why Climate Change Is Shifting the Risk
Rising temperatures favor bacterial pathogens. A large review of 141 studies found that for every degree of temperature increase, bacterial diarrhea incidence rose about 7%. Viral diarrhea showed no similar link, suggesting that heat specifically boosts the survival or transmission of bacteria in water. More frequent and intense flooding flushes pathogens into water supplies, while longer droughts concentrate those same pathogens in shrinking sources.
This pattern has already played out in real populations. During an El Niño warming event in the 1990s, hospitals in Lima, Peru, saw a measurable spike in diarrheal admissions. As extreme weather events become more common, both wealthy and low-income countries face growing pressure on their water treatment systems.
How Public Water Systems Remove Pathogens
Municipal water treatment relies on disinfection to kill germs before water reaches your tap. The two most common disinfectants are chlorine and chloramine, added at low levels during treatment. Chlorine works fast but breaks down relatively quickly in long pipe networks. Chloramine is slower-acting but lasts longer, continuing to kill germs as water travels through miles of distribution pipes. Some utilities switch temporarily from chloramine back to chlorine to strip biofilm buildup from pipe walls, which helps control Legionella and similar organisms.
U.S. federal regulations set the maximum contaminant level goal at zero for the most dangerous waterborne pathogens: Giardia, Cryptosporidium, Legionella, E. coli, and viruses. Zero is an aspirational target, not always achievable, but it drives utilities to treat water as aggressively as possible. Cryptosporidium poses a particular challenge because its hard outer shell resists chlorine, so many treatment plants use additional filtration or UV light to address it.
Protecting Yourself at Home and While Traveling
Boiling is the single most effective way to make water safe from biological contamination. A rolling boil for one minute kills viruses, bacteria, and parasites. At elevations above 6,500 feet, where water boils at a lower temperature, extend that to three minutes. Boiling does not remove chemical contaminants or radioactive material, so it is specifically a defense against germs.
Other proven household methods include adding chlorine (sold as water purification drops or tablets in outdoor and travel stores), using ceramic or membrane filters rated for the pathogen size you’re concerned about, and solar disinfection. Solar disinfection works by placing water in a clear container in direct sunlight for six to eight hours on a sunny day, or two days under cloud cover. The combination of UV radiation and heat kills most bacteria and parasites, though it is less reliable against all viruses.
When traveling to regions with uncertain water quality, these same principles apply: drink boiled or bottled water, avoid ice made from tap water, and be cautious with raw foods washed in local water. In your home country, if your water comes from a private well rather than a municipal system, regular testing is the only way to know whether pathogens have entered the supply, since private wells are not covered by federal drinking water regulations.
Who Faces the Greatest Risk
Children under five bear the heaviest burden worldwide. Diarrheal disease is the third leading cause of death in children ages one to 59 months, with roughly 1.7 billion episodes of childhood diarrhea occurring globally each year. The vast majority of these cases trace back to contaminated water or food in settings where sanitation infrastructure is minimal. As of 2022, 27% of the world’s population still lacked safely managed drinking water, defined as water that is available at home, accessible when needed, and free from contamination.
People with compromised immune systems face elevated risk regardless of where they live. Cryptosporidium, for instance, can cause prolonged and dangerous illness in people with HIV, transplant recipients on immune-suppressing drugs, and those with inherited immune disorders. For these individuals, even treated municipal water may carry enough residual risk to warrant additional home filtration or boiling during outbreak advisories.