The signs and symptoms of HUS may vary, depending on the cause. Most cases of HUSare caused by infection with certain strains of E. coli bacteria, which first affect the digestive tract. The initial signs and symptoms of this form of HUS may include:

• Diarrhea, which is often bloody
• Abdominal pain, cramping or bloating
• Vomiting
• Fever

All forms of HUS — no matter the cause — damage the blood vessels. This damage causes red blood cells to break down (anemia), blood clots to form in the blood vessels and kidney damage. Signs and symptoms of these changes include:

• Pale coloring, including loss of pink color in cheeks and inside the lower eyelids
• Extreme fatigue
• Shortness of breath
• Easy bruising or unexplained bruises
• Unusual bleeding, such as bleeding from the nose and mouth
• Decreased urination or blood in the urine
• Swelling (edema) of the legs, feet or ankles, and less often in the face, hands, feet or entire body
• Confusion, seizures or stroke
• High blood pressure

The most common cause of HUS — particularly in children under the age of 5 — is infection with certain strains of E. coli bacteria. E. coli refers to a group of bacteria normally found in the intestines of healthy humans and animals. Most of the hundreds of types of E. coli are normal and harmless. But some strains of E. coli cause diarrhea.

Some of the E.coli strains that cause diarrhea also produce a toxin called Shiga toxin. These strains are called Shiga toxin-producing E. coli, or STEC. When you are infected with a strain of STEC, the Shiga toxin can enter your bloodstream and cause damage to your blood vessels, which may lead to HUS. But most people who are infected with E. coli, even the more dangerous strains, don't develop HUS.

Other causes of HUS can include:

• Other infections, such as infection with pneumococcal bacteria, human immunodeficiency virus (HIV) or influenza
• The use of certain medications, especially some of the medications used to treat cancer and some of the medications used to suppress the immune system of organ transplant recipients
• Rarely, HUS may occur as a complication of pregnancy or health conditions such as autoimmune disease or cancer

An uncommon type of HUS — known as atypical HUS — can be passed down genetically to children. People who have inherited the mutated gene that causes this form of HUS won't necessarily develop the condition. But the mutated gene might be activated after exposure to a trigger, such as an infection, the use of certain medications or a chronic health condition.

The majority of HUS cases are caused by infection with certain strains of E. coli bacteria. Exposure to E. coli can occur when you:

• Eat contaminated meat or produce
• Swim in pools or lakes contaminated with feces
• Have close contact with an infected person, such as within a family or at a child care center.

The risk of developing HUS is highest for:

• Children 5 years of age or younger
• Adults 65 years of age or older
• People who have a weakened immune system
• People with certain genetic changes that make them more susceptible to HUS

HUS can cause life-threatening complications, including:

• Kidney failure, which can be sudden (acute) or develop over time (chronic)
• High blood pressure
• Stroke or seizures
• Coma
• Clotting problems, which can lead to bleeding
• Heart problems
• Digestive tract problems, such as problems with the intestines, gallbladder or pancreas