Former President George H.W. Bush is being treated for pneumonia in intensive care at a Houston hospital where doctors are evaluating him before removing a breathing tube.

His underlying health problems include vascular parkinsonism, a rare syndrome that mimics Parkinson's disease. The 92-year-old Bush also broke a vertebra in 2015 and has used a motorized scooter or a wheelchair in recent years. Some answers to common questions about his health:

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Q: WHEN DOES A PNEUMONIA PATIENT NEED A BREATHING TUBE?

A: When a patient is not getting enough oxygen in the blood, doctors will insert a soft plastic breathing tube into the windpipe and connect the patient to a breathing machine called a ventilator. The patient is usually sedated because inserting the tube is uncomfortable.

"It means it's a really bad pneumonia and is certainly life-threatening," said Dr. Michael D. Schwartz, a pulmonologist and critical care doctor at National Jewish Health in Denver, Colorado. Doctors at the patient's bedside "believe this person is likely to perish without this type of life support."

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Q: WHEN CAN THE BREATHING TUBE COME OUT?

A: The longer a patient has a breathing tube, the higher the risk of a secondary infection. So when an improving patient can follow simple commands and has a strong enough cough, doctors will remove the tube and let the sedation wear off.

Coming off sedation can bring on delirium, especially for elderly patients, so doctors will monitor whether the patient is agitated and disoriented, Schwartz said.

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Q: WHAT ARE THE RISKS OF PNEUMONIA?

A: Pneumonia is an infection of the lungs that can be mild or severe. Elderly patients are at risk of deadly complications.

The former president wrote to President-elect Donald Trump on Jan. 10, saying that he would be unable to attend Friday's inauguration because of doctor's orders: "My doctor says if I sit outside in January, it will likely put me six feet under. Same for Barbara. So I guess we're stuck in Texas."

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Q: WHAT IS VASCULAR PARKINSONISM?

A: People diagnosed with the condition walk with shuffling steps, and brain scans suggest they have suffered small strokes. However, they do not have the characteristic tremors of Parkinson's disease, and they do not respond to drugs for Parkinson's.

"They look like Parkinson's from the waist down. From the waist up, they look very expressive," said Dr. Alberto Espay of the University of Cincinnati's Gardner Neuroscience Institute.

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Q: IS IT DIFFERENT FROM PARKINSON'S DISEASE?

A: Yes. It does not get steadily worse in the same way Parkinson's does. Classic Parkinson's disease develops when cells that produce one of the brain's chemical messengers, called dopamine, begin to deteriorate and die. Dopamine transports signals to parts of the brain that control movement. Parkinson's symptoms appear after enough dopamine-producing cells die that there's too little of this neurotransmitter in the brain.

Vascular parkinsonism can closely mimic a number of other disorders, including classic Parkinson's, progressive supranuclear palsy and excessive fluid on the brain.

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Q: WHAT HAS PRESIDENT BUSH SAID ABOUT THE CONDITION?

A: In a 2012 interview with Parade magazine, Bush said: "It just affects the legs. It's not painful. You tell your legs to move and they don't move. It's strange, but if you have to have some bad-sounding disease, this is a good one to get."

Bush said the symptoms have been difficult for him, "because I love being active, (playing) sports, being in the game. But you just face the reality and make the best of it."

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Q: WHO GETS IT?

A: People in their 70s and older are most likely be diagnosed, said Dr. Tanya Simuni who conducts research on Parkinson's and other movement disorders at Northwestern Medicine in Chicago. She calls it a "difficult diagnosis" because of the lack of accurate diagnostic tests.

The risk factors are the same as for stroke and heart disease: history of smoking, lack of exercise and poor diet. It's rarer than Parkinson's disease, which affects about 1 million Americans. In contrast, roughly 20,000 people have been told they have vascular parkinsonism.

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Q: HOW IS IT TREATED?

A: Since the condition does not respond well to drugs used to treat Parkinson's disease, treatment relies on lowering the risk factors for stroke. That means quitting smoking, exercising, eating a healthy diet and perhaps taking a daily aspirin. Preventing falls is important, so a patient may receive physical therapy to improve balance.

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Q: WHAT CAUSES THE DISEASE?

A: This is where answers get tricky. Conventional thinking says the disease is caused by strokes. But without a perfect test, lots of problems get lumped under the umbrella. Espay has written a paper questioning whether most parkinsonism is related to vascular disease.

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Q: IS THERE A CURE?

A: Physical therapy can help with balance and walking, but the damage cannot be reversed, said Dr. Corneliu Luca, assistant professor of neurology at University of Miami. "If you do not control risk factors for stroke, they can have another stroke, and the walking can get even worse," Luca said.

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Q: WHAT COMPLICATIONS ARE MOST WORRISOME?

A: Falls are the biggest concern because they can lead to fractures, Simuni said. Bush took a spill in 2015 and fractured his C2 vertebra, the second one below the skull. It's not known whether that fall was directly related to parkinsonism.