Bill Clinton is reportedly hospitalized for sepsis after suffering a UTI. Here's what that means.

• Bill Clinton is being treated for a UTI that progressed into sepsis, and is "on the mend."
• The complication is relatively common in older age, and can be treated with antibiotics and fluid.

Former President Bill Clinton is being treated for sepsis after a urinary tract infection spread to his bloodstream, multiple media outlets have reported.

The complication is common, especially among older people, but can be dangerous if left untreated.

Clinton, 75, is being closely monitored in the intensive care unit, where he's received IV antibiotics and fluids, according to a statement from his physicians.

On Thursday night, his spokesperson said he's "on the mend, in good spirits, and incredibly thankful to doctors, nurses, and staff providing him with excellent care."

Here's what to know about UTIs and sepsis.

UTIs are more common in women than men

UTIs are caused when bacteria gets into the urinary system, typically in the bladder and urethra. That can happen due to tight clothing, poor bathroom or post-sex habits, dehydration, or other issues.

They're more common in women than men due to their shorter urethras, which makes it easier for bacteria to sneak in from the rectum. About one in 5 women experiencing them in their lifetime compared to the 3% of men worldwide who get one each year.

While they don't always cause symptoms, UTIs can lead to an urge to urinate, a burning sensation when peeing, unusual looking or smelling urine, and pelvic pain.

Sometimes the body flushes the infection out naturally, but other times it needs to be treated with antibiotics and fluids. If not, it can spread to other organs or the blood and become dangerous.

If the body or antibiotics don't clear the infection, it can spread and become dangerous

NBC News senior medical correspondent Dr. John Torres told Today that it's common for older people's bodies to be unable to "contain" the infection, causing it to move into the bloodstream. At that point it's called "bacteremia," Torres said.

If the infection continues to progress, it becomes sepsis, which is "basically the body's response to that overwhelming infection where things start not working as well as we'd like them to," Torres said. Organs like the kidneys, liver, and brain start to dysfunction.

If untreated at that point, or if treatments don't work, sepsis can lead to septic shock, a deadly complication in which the organs shut down.

"People shouldn't die from a UTI, but if sepsis begins to take over and develops to severe sepsis and then to septic shock, this is exactly what can happen," the Sepsis Alliance writes.

Clinton's infection seems to have been controlled at a much earlier stage. His doctors said he's responding to antibiotics and that his white blood cell count - a marker of infection - is "trending down."