Previous Internet chatter about Castro includes an August 2011 spam email titled "Fidel is Dead" that had a virus embedded.
In Miami, the rumors circulated among friends, on the radio and on television.
Vicente Rodriguez, editor of La Voz de la Calle newspaper, said the exile community was monitoring the gossip with guarded optimism, after having heard them many times before.
"This is like when someone at a beach starts yelling, 'I'm drowning,' and then they say, 'No, it's a joke,'" Rodriguez said. "We never know whether we're on the precipice of truth or another trick of the regime."
Associated Press writers Ian James in Caracas, Venezuela, and Gisela Salomon and Christine Armario in Miami contributed to this report.
Associated Press writer Peter Orsi on Twitter: www.twitter.com/Peter_Orsi