From the moment the waters began rising in Texas last month, disease was on health officials’ minds. Floodwaters, after all, are filthy.

When Hurricane Harvey finally moved north and the feet of flooding drained, hospitals saw a spike in skin and gastrointestinal infections, but Texans were spared some of the most serious illnesses that contaminated water can spread: cholera, for instance, and typhoid.

On Tuesday, however, the Harris County medical examiner’s office announced that the death of a 77-year-old woman 11 days earlier had been caused by necrotizing fasciitis: a gruesome and often deadly infection commonly known as flesh-eating bacteria.

The woman, Nancy Reed, contracted the disease when she fell inside a flooded home in Houston’s Kingwood community and broke her arm, allowing bacteria from the floodwaters in through cuts. Hers was the 36th storm death recorded in Harris County.

Porfirio Villarreal, a spokesman for the Houston Health Department, said the city had received no other reports of necrotizing fasciitis since the storm. But a nonfatal infection was confirmed in nearby Missouri City, Tex., where J. R. Atkins, a former firefighter and paramedic, contracted the disease while helping neighbors escape the floodwaters. In Mr. Atkins’s case, The Houston Chronicle reported, the bacteria entered through an insect bite on his arm.

Another person, Clevelon Brown of Galveston County, died not of necrotizing fasciitis but of sepsis caused by a different bacteria from the floodwaters.

In necrotizing fasciitis, bacteria infect the fascia, a type of connective tissue. The bacteria can produce toxins that destroy the tissue. (They do not eat it, despite the “flesh-eating bacteria” label.)

If caught early enough, the infection can be treated with antibiotics and surgery. But many patients lose limbs, and even with treatment, 25 to 35 percent die.

A wide variety of bacteria can cause the disease. The most common is group A streptococcus, the same germs responsible for strep throat. This is the only type of necrotizing fasciitis that is formally tracked, and 700 to 1,100 cases are recorded in the United States each year, according to the Centers for Disease Control and Prevention. Other culprits include E. coli, Staphylococcus aureus, Aeromonas hydrophila, and members of the Clostridium and Klebsiella genera.

When The New York Times organized testing in Houston after the hurricane, floodwaters in two neighborhoods showed extremely high levels of E. coli: in one home, 135 times the level considered safe.

The sample in that home also tested positive for Vibrio bacteria. It was not possible to identify the specific strain, but one — Vibrio vulnificus — is known to cause necrotizing fasciitis, said Charlotte Smith, an environmental health expert at the University of California, Berkeley’s School of Public Health who helped The Times analyze the data.

Dr. Smith said she was not surprised that necrotizing fasciitis infections had occurred. Cases were also recorded after Hurricane Katrina, she noted, because of the confluence of two factors.

“One is the breakdown of the infrastructure as a whole, which brings more microbes into the environment,” Dr. Smith said. “The second is the conditions of people that are in that environment: They have high potential for cuts and scrapes that would make one more susceptible.”

Activities like taking furniture to the curb or removing drywall can easily cause the cuts and abrasions that allow bacteria in.

The good news is that necrotizing fasciitis is extremely rare, even in people who are exposed to the bacteria that can cause it. Most people who contract it have compromised immune systems or underlying conditions like diabetes, kidney disease or cancer, the C.D.C. says.

The easiest way to prevent infection is to clean wounds, even superficial ones, immediately and keep them covered until they heal. The C.D.C. also recommends that people with open wounds avoid water: lakes, oceans, pools and hot tubs alike.

But, of course, flooding as catastrophic as what the Houston area experienced makes that impossible. Dr. Smith said coveralls, gloves and boots were key precautions to take when working in or around contaminated water.

Early symptoms — like warm skin, swelling, and red or purplish coloration in the affected area — can be difficult to distinguish from those of less serious infections, though the C.D.C. says patients “often describe their pain as severe and hurting much more than they would expect based on how the wound looks.”

But a hallmark of necrotizing fasciitis is rapid progression, evident in the spread of skin redness, said Dr. David Persse, physician director of Houston’s emergency medical services.

“Most wound infections will grow at a slow pace where you notice the change from one day to the next,” Dr. Persse wrote in an email. “With necrotizing fasciitis, you will notice the infection spreading over just a few hours.”

This makes immediate treatment essential. Dr. Persse noted that Mr. Atkins, the Missouri City man who survived an infection, recognized the symptoms quickly because of his background as a paramedic and sought emergency care right away.