Bill Clinton's hand appears to tremble as he campaigns for Hillary in Iowa

Bill Clinton's hand appeared to tremble as he campaigned for his wife in Iowa, raising questions about his health.

Observers noticed that his fingers appeared to shake as he delivered a speech over the weekend to a crowd of 5,000. In the past, he's said it's simply a condition of aging.

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In 2013, Clinton denied he had Parkinson's disease, saying: "I have a condition that sometimes you get with aging. You may have noticed it; my hand has a little tremor when I'm tired and a lot of people do when they're older."

See recent photos of Bill Clinton on the campaign trail:

During the 69-year-old's speech, he explained why his wife partnered with Wall Street after the 9/11 attacks.

He said: "She was making the point that the people devastated on 9/11 were people to her, not categories."

Bill Clinton was also a topic during the Democratic debate on Sunday night. Senator Bernie Sanders was asked to explain his recent comments about the former president.

Moderator Andrea Mitchell asked Sanders: "You've called Bill Clinton's past transgressions 'totally, totally disgraceful and unacceptable.' Senator, do you regret saying that?"

See Bill and Hillary Clinton through the years:

Sanders hit back: "That question annoys me... Yes, his behavior was deplorable. Have I ever once said a word about that issue? No, I have not. I'm going to debate Secretary Clinton, Governor O'Malley, on the issues facing the American people, not Bill Clinton's personal behavior."

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Hillary Clinton only discussed what role her husband would play in her administration, saying: "Well, it'll start at the kitchen table, we'll see how it goes from there."

She added: "I'm going to use him as a goodwill emissary to go around the country to find the best ideas we've got, because I believe, as he said: 'Everything that's wrong with America has been solved somewhere in America.'"

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