HAVANA — It's been more than a year since Fidel Castro burst back on the scene with a spate of public appearances and dire warnings of nuclear Armageddon. But after a flurry of activity that quieted speculation about his exit from the world stage, the Cuban revolutionary's revival tour seems to be over.

Castro has not appeared in public since a key Communist Party meeting in April when he seemed unsteady and unusually frail. He has also virtually stopped writing his trademark opinion pieces and didn't make a statement or release a photograph on his 85th birthday in August.

The silence has prompted the usual death rumors from Miami, propagated on exile radio and television stations and through social media sites such as Twitter. Castro's health has even been the subject of a computer virus embedded in a spam email titled "Fidel is Dead," which features a doctored, grainy photograph that appeared to show the Cuban leader lying in a coffin.

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In Venezuela, a newspaper claimed Fidel's supposedly failing health explained why President Hugo Chavez remained in his home country for a third round of chemotherapy, after receiving treatment in Havana on the first two occasions.

The Cuban government, as always, has remained silent. Requests by The Associated Press for comment on Castro's health and on what he does with his days went unanswered. The government keeps his exact whereabouts a state secret and has long kept even mundane details of his personal life private.

"My premise with Fidel Castro is you start with the fact that he's Lazarus and proceed from there," said Ann Louise Bardach, the author of "Without Fidel: A Death Foretold in Miami, Havana and Washington," which she began writing in 2006 to coincide with Castro's much-predicted demise.

"Whatever you think of him, this man has a life force which is formidable," Bardach said. "We're not dealing with a normal mortal here. If there is ever going to be somebody who never dies, it's him."

Transfer of power
Castro stepped down in July 2006 and turned over power to his brother Raul due to a serious intestinal illness that he later said nearly killed him. He continued to publish opinion pieces, called "Reflections," in state newspaper Granma but remained out of the public eye for four years before suddenly reappearing in July 2010. He met then with economists, diplomats and lawmakers and even attended a dolphin show at the Havana aquarium.

Before long, Castro was back rallying throngs of supporters under the Havana sun and had dusted off his olive-green military fatigues. He seemed to soak up the attention.

He used his return to the limelight to warn about the threat of a nuclear exchange pitting the United States and Israel against Iran. Later, as Arab Spring protests roiled both pro- and anti-Western governments in the Middle East, Castro showed solidarity with longtime ally Moammar Gadhafi by publishing biting criticism of NATO and the United States. He wrote that the intervention in Libya was a "macabre dance of cynicism" designed to seize Libyan oil fields.

In April, Castro relinquished his final official role as Communist Party chief, making a dramatic appearance with his brother at the close of a key party gathering. Castro needed the help of a young aide to walk to his chair as hundreds of delegates applauded, some with tears in their eyes. Once seated, he seemed to slump over as if losing his balance.

Since then, Castro has made no public appearances, though photographs and video of him meeting with Chavez and other visiting dignitaries have been released. His absence has been noted on the streets.

"He seemed so fragile to me the last time he was on television, when we saw him with Chavez," said Angela Blanco, a 66-year-old Havana resident. "I'm worried about him because he used to be so strong."

Waiting on Cuba's confirmation
Most surprisingly, the Cuban revolutionary has stopped writing about world affairs that are clearly close to his heart. He said nothing last month as Gadhafi's forces succumbed to a rebel offensive and the Libyan leader went into hiding. He hasn't commented on the recent violence in Syria, the London street riots in August or the usual political bickering in Washington, all of which would have certainly drawn punditry from a more active Castro.

In fact, the Cuban leader has published just one opinion piece since May 26, announcing that Chavez's first round of chemotherapy was a success.

So far this year, Fidel has published his thoughts or statements 37 times, compared with 85 times in 2010, and 111 the previous year, according to an AP count.

While he has gone on hiatus before, Castro's most recent silence, coupled with his milestone 85th birthday, has increased the sense that one of the most energetic figures of the 20th century is slowing down.

Castro himself alluded to his limitations in an apology to well-wishers after missing outdoor celebrations in April marking the 50th anniversary of Cuba's victory at the Bay of Pigs.

"Believe me that I felt pain when I saw that some of you were looking for me on the dais," he wrote. "I thought everyone understood that I can no longer do what I have done so many times before."

Still, Cuban-American observers have learned from experience to question whispers that Castro's end is near.

"When you've been following this as long as I have, you don't believe any news about Castro's health until you read the confirmation from the Cuban government," said Tomas Bilbao, executive director of the U.S. Cuba Study Group, a business-backed organization that supports exchanges with the island. "As far as I'm concerned, nothing's changed."