(CNN) -- Flesh-eating bacteria are common in the environment, but they rarely infect humans.
When they do cause an infection, they attack the body quickly, and doctors must act immediately to prevent their spread.
Many different types of bacteria cause the condition known as necrotizing fasciitis, in which the bug attacks healthy tissue and destroys it.
One of the bacteria is Aeromonas hydrophila, which caused the infection in Georgia student Aimee Copeland. The bacteria entered her body through a gash in her leg, which she suffered in a May 1 zip line accident. Doctors have already removed part of her abdomen, amputated a leg, and expect to remove her fingers to try to halt its spread.
Another is Group A Strep, the same bacteria that cause strep throat.
Many other bacteria -- especially those that thrive in oxygen-deprived environments, like inside the human body -- can also cause necrotizing fasciitis, said Dr. William Schaffner, chairman of the Department of Preventive Medicine at the Vanderbilt University Medical Center.
The bacteria that can cause the condition exist normally in the environment and are even found in the body, but a person's immune system is usually able to fight off infection. Group A Strep, for instance, is often in the throat but exists in a small enough number not to cause problems.
Even if swallowed, the worst a person may experience is vomiting or diarrhea.
Occasionally, however, the bacteria find their way into the bloodstream. That can happen through a cut or abrasion, as was the case with Copeland.
But sometimes blunt trauma can lead to infection. If the trauma causes a bit of internal bleeding, even just a black-and-blue mark, the bacteria inside the body sees the blood and decides to attack it. They lodge in the damaged tissue and quickly spread, Schaffner says.
Such was the case with South Carolina woman Lana Kuykendall. After delivering twins May 7 and returning home, she noticed a bruise on the back of her leg that grew in size as she watched, her husband said.
Doctors have cut out skin and tissue from both of Kuykendall's legs and put her on antibiotics. She is in critical condition and on a ventilator, but her husband said she will likely be OK.
Cases of necrotizing fasciitis are rare. Schaffner estimates fewer than 250 cases occur every year, though it is impossible to say for sure because by law the reporting of such cases is not required.
Researchers only recently began trying to collect numbers, the FDA says.
In addition, medical professionals are so good at treating wounds that often, even if the body is infected with flesh-eating bacteria, the wound is cleaned up and rid of the bug before it does any damage, Schaffner says. Necrotizing fasciitis is never diagnosed in such cases.
When doctors do diagnose the condition, they must act quickly. Surgeons must cut out more tissue than they would otherwise -- cutting even healthy tissue to go beyond the visible infection.
And after surgery, surgeons must repeatedly check the area to see whether the infection has spread.
Schaffner says people who suffer cuts should pay attention to any pain coming from a closed wound, as well as redness or drainage, and report such symptoms to a doctor right away.