There’s nothing better on a hot and humid day than wading into a lake or river to cool off, but recent E. coli outbreaks raise questions about water safety.

Swimming in natural waters is never completely risk-free, so state and local environmental agencies monitor beaches, lakes and rivers and routinely publish test results.

Two dozen people got sick with E. coli infections last month after swimming at Lake Anna in Virginia, and the Anacostia Riverkeeper on Saturday postponed again the first sanctioned swim in the river in a half-century because of elevated E. coli levels.

Here’s a guide to what governments are doing to monitor water quality for safe swimming.

What is E. coli, and what are the symptoms of an infection?

E. coli, or Escherichia coli, bacteria are used as indicators of recreational water quality because they are easy to detect and commonly found in animal and human waste. Most strains are harmless, but some can cause infections that make people sick with stomach cramps, vomiting, fever, chills and diarrhea that is often watery or bloody.

This happens when someone ingests food or water contaminated with a small, often invisible, amount of animal or human feces, according to the Centers for Disease Control and Prevention.

Children may develop a severe complication of an E. coli infection known as hemolytic uremic syndrome, or HUS, which can cause kidney failure, according to a website on the Virginia outbreak.

How often are lakes and rivers tested for bacteria?

In Maryland, local health departments determine where, when and how often water samples are collected at beaches. But this usually means samples are taken as often as weekly or as infrequently as monthly, depending on how much use they get and the potential risk of infection.

Samples are sent to the Maryland Department of Health lab for analysis. When samples exceed fecal bacteria standards, the results are reported to local health departments, which warn the public there is an elevated risk that swimmers may get sick.

The D.C. Department of Energy and Environment tests for E. coli at 30 locations in the Potomac and Anacostia Rivers, Rock Creek and their tributary streams at least monthly throughout the year and weekly between May and September. These three main water bodies have routinely fallen below water quality standards established to protect humans, fish and wildlife, according to the agency.

How much E. coli is too much?

E. coli is measured in terms of MPN (or most probably number) of bacteria cells per 100 milliliters.

In D.C., officials say the safe level of E. coli for short-term exposure to the water is 410 MPN per 100 mL. The standard for a water body to be considered “swimmable” over the long-term is an average of 126 MPN per 100 mL over a 30-day period.

Virginia’s Water Quality Standards use the same standard but for over a 90-day period.

Virginia and Maryland officials say, based on Environmental Protection Agency studies, there is an elevated risk of illness associated with swimming in natural waters when the E. coli level exceeds 235 counts per 100mL.

Where can I see test results?

In Maryland, local health departments conduct testing of waters off beaches and lakes. Results are posted at the “Check My Beach” tab on the state Department of the Environment website.

D.C. water quality data is available from the state’s Water Quality Dashboard. Volunteer water quality data is also available online.

In Virginia, the Department of Environmental Quality’s monitoring data are available to the public through its portal. The agency’s annual monitoring plan includes an interaction map of sampling stations.

Does climate change have anything to do with E. coli in the water?

D.C. officials note bacteria levels do increase as temperatures rise. But the biggest influence on bacteria levels in natural waters is rainfall, which flushes pollution, including E. coli, into rivers and streams.

“We don’t fully understand the impact climate change will have on local water quality. Climate change could potentially cause more intense storms, which would generate more stormwater runoff and associated pollution,” according to a statement from the city’s water quality team.

Maryland officials, however, noted warm water temperatures can impact the prevalence of another bacteria, Vibrio, which occur naturally in brackish water such as the Chesapeake Bay and its tributaries and in salt water. Vibrio infections are relatively rare in Maryland and nationwide, but can be serious.

Are E.coli levels this summer higher than usual?

Not according to District environmental experts. Sources of E. coli include wildlife and sanitary sewage and these can vary by time of year and with the weather.

With the recent completion of the Anacostia Tunnel portion of the Clean Rivers Project, city officials expect E. coli levels to improve, especially following heavy rains that have historically caused overflows from the city’s combined sewer.

What impacts water quality?

Water quality can deteriorate due to pollution from runoff after storm events, trash, debris or sewage, the Maryland Department of the Environment says. Sewage sources include bypasses from sewage pumping stations, combined stormwater sewers and sewage spills.

Poor water quality at beaches could also be caused by failing septic systems, boat waste discharges, and wastes from pets, wildlife and farm animals that may runoff into the waters after storms.

What safety measures should swimmers consider?

Local health departments offer these tips for people who are visiting natural water sites: