The aptly named Goodluck Jonathan took over as acting president of Nigeria this week, filling the vacuum left by the long absence of President Umaru Yar'Adua. It's not as unusual as you might think: other heads of state have been ill, have died, or have simply not been there much of the time.
But let's begin with the case of Nigeria today:
1. President Umaru Yar'Adua (Nigeria)
Yar'Adua has not been seen in public since he went to Saudi Arabia for medical treatment on 23 November 2009. The Nigerian constitution requires the president to inform parliament formally of his absence before power can be transferred -- but he didn't do this before he flew out of Abuja, leaving his country leaderless.
Conspiracy theories abounded, variously that he was dead, brain-dead, or no longer in hospital. The past three months have been fraught with legal challenges, cabinet splits, mass protests, and tensions resurfacing in the Niger Delta. Eventually, the National Assembly voted in favour of Vice-President Goodluck Jonathan stepping in. It remains to be seen how successful he will be in ending the turmoil.
2. Kim Jong-il (North Korea)
The whereabouts of the elusive North Korean leader have been the subject of heavy speculation since 2008. In August that year, Toshimitsu Shigemura, a Japanese academic and expert on North Korea, claimed that the Great Leader had died in late 2003 and been replaced at public appearances by stand-ins, previously employed to protect him from assassination attempts.
On a slightly less far-fetched note, questions about Kim's health were raised when he failed to appear at a military parade for the 60th anniversary of the founding of North Korea, on 9 September 2008. US intelligence agencies believed that he could be "gravely ill" after suffering a major stroke, although Pyongyang downplayed this.
Various reports have suggested that he has since suffered a second stroke, or that he has pancreatic cancer. The Times has questioned the veracity of at least one photograph released to prove that he is still alive. However, Kim met with Bill Clinton in August last year to negotiate the release of two Americans. No doubt conspiracy theorists would say that Clinton met with one of his impersonators.
3. Fidel Castro (Cuba)
The long-time Cuban leader handed over power to his brother, Raúl, in July 2006, and resigned from the presidency in 2008. But this followed years of speculation about Castro's health, beginning in 1998 with the rumour -- since discredited -- that he has a terrible brain disease.
The belief that his health was failing gathered force in 2004, when the mayor of Bogotá said that he "seemed very sick to me", and the following year when the CIA said he had Parkinson's disease (which he denied).
Since then, it has been widely accepted that he is seriously sick, with some speculating that he has a terminal illness. On reports of his recovery, George W Bush said: "One day the good Lord will take Fidel Castro away." Although he has officially retired from public affairs, Cubans still worry that he might be dead.
4. Lansana Conté (Guinea)
Conté became Guinea's leader by staging a coup d'état in 1984. On falling ill in 2006, he was reluctant to relinquish power, creating a leadership vacuum that led to an outbreak of violence. In April 2006, he was flown to Morocco for medical treatment, amid riots in Guinea over the price of rice and fuel. He returned, but was physically and mentally incapacitated, leaving the door open for his wife, Henriette, to flout the rule of law.
Despite his illness, Conté insisted he would stay in office until the end of his term in 2010. A general strike brought the country to a halt for two weeks in 2007, hundreds of thousands of workers taking to the streets to protest against his leadership.
Many died in violent clashes with security forces. The strike came to an end when Conté agreed to appoint a new prime minister, Lansana Kouyaté, as acting head of government. Yet he declared on television that year: "I'm the boss, others are my subordinates." He then undermined Kouyaté and soon replaced him with another proxy.
Conté died of natural causes in December 2008. Within hours of the new prime minister announcing his death, junior army officers staged a coup, bringing Captain Moussa Dadis Camara to power. His junta failed to bring stability to Guinea. Last September, a contingent of presidential guardsmen massacred more than 150 opposition supporters at a rally in the capital, Conakry. Then, in December, an aide to Camara shot him in the head; he was flown out to Morocco for medical treatment.
On 13 January, after an absence of six weeks, Camara left Morocco for Burkina Faso. He is yet to return from there to Guinea, but in his absence an official investigation declared him free of all responsibility for the September massacre.
5. Saparmurat Niyazov (Turkmenistan)
And finally, a potential crisis of leadership that ended well. You might not have heard of Saparmurat Niyazov, the Turkmen dictator, who was compared to Stalin and -- yes -- Kim Jong-il for his draconian rule.
Criticised internationally as one of the world's most repressive dictators, he encouraged a bizarre cult of personality, renaming months of the year after members of his family, and replacing the word for bread with his mother's name. He also banned the opera and the circus for not being Turkmen enough.
When he died in December 2006, the oil- and gas-rich central Asian nation was left in turmoil as essentially he was the entire state. Happily, fears that the vacuum created by his death would lead to a power struggle between different regions, or even civil war, were not realised. Vice-President Gurbanguly Berdimuhamedow took over, repealing some of Niyazov's stranger policies, and tentatively opened up relations with the west.