Some people might link UTIs to discomfort during or after urination. These symptoms usually indicate a “simple” or “uncomplicated” infection, especially if no other symptoms are present. Complicated UTIs typically cause flu-like symptoms that affect the whole body.

“UTI” is the broad name for any bacterial infection that affects some part of the urinary tract, explains Victoria Scott, MD, urologist and medical advisor at Flo Health.

Many UTIs affect just the lower portion of the urinary tract, which comprises the urethra and bladder. This type of infection, known as a lower UTI, is easily treatable with antibiotics.

Some UTIs affect the upper portions of the urinary tract, like the kidneys or ureters. These UTIs — classified as upper UTIs — are treatable. However, upper UTIs are usually more complicated to treat than lower UTIs.

Scott explains: Your kidneys filter your blood. So, when a bacterial infection like a UTI infects your kidneys, there’s a risk that the infection enters the blood and causes sepsis.

Scott says people with upper or complicated UTIs often experience full-body symptoms, such as:

“In some cases, people will also experience nausea or vomiting so severe that you cannot keep down liquids or food,” explains Scott.

Many people with upper UTIs will also experience the symptoms of lower UTIs — but not always, she says.

These symptoms may include:

The biggest risk factor for an upper UTI is a lower UTI that’s undiagnosed and untreated. Although a lower UTI can resolve without treatment, it’s unlikely.

“If a UTI does not get treated and does not resolve by itself, it can progress to the upper urinary tract and cause a kidney infection,” says Scott

Certain circumstances or underlying conditions can also increase the risk of a complicated UTI. For example, some experts consider UTIs that develop during pregnancy or in people with a penis complicated by nature.

The following can heighten the risk of UTI — and if a UTI occurs, experts may consider it complicated — in people of all anatomies:

Experts also may consider recurrent UTIs and antibiotic-resistant UTIs complicated.

“As soon as you develop any symptoms of a complicated UTI, you should seek care and get antibiotics,” says Scott.

Upper or complicated UTIs need treatment to clear up and prevent them from worsening.

Without getting medical care when you have an upper UTI, in particular, there’s a risk that the infection leads to sepsis, says Aleece Fosnight, a board certified physician assistant and medical adviser at Aeroflow Urology.

“Sepsis is a very serious medical complication that needs to be treated with IV antibiotics and closely monitored with admission to a hospital,” explains Fosnight. Getting treatment for sepsis can prevent death.

Upper or complicated UTIs generally cause symptoms that affect the whole body. These include fever, chills, overall weakness, and side pain.

If you have unusual symptoms or suspect that you have a UTI, it’s important to consult with a healthcare professional. They can determine the underlying cause and make or confirm a UTI diagnosis.

Gabrielle Kassel (she/her) is a queer sex educator and wellness journalist who is committed to helping people feel the best they can in their bodies. In addition to Healthline, her work has appeared in publications such as Shape, Cosmopolitan, Well+Good, Health, Self, Women’s Health, Greatist, and more! In her free time, Gabrielle can be found coaching CrossFit, reviewing pleasure products, hiking with her border collie, or recording episodes of the podcast she co-hosts called Bad In Bed. Follow her on Instagram @Gabriellek