Nigerian President Muhammadu Buhari was elected in 2015 on a wave of hope that he could defeat Boko Haram Islamists and turn around a country blighted by decades of corruption and poor governance.But while he will go down in history as Nigeria's first opposition candidate to defeat an incumbent, his first term was dominated by questions about his fitness to govern.From May 2016 until mid-2017, Buhari was in London for medical treatment for increasingly long periods of time, forcing government denials that he was gravely ill or even dead.To date, he has not disclosed details about his condition, apart from saying he had "never been so ill" and had to undergo multiple blood transfusions.But after US President Donald Trump reportedly called him "lifeless" after the recovering Buhari visited Washington, his critics found a potent slur.His absence also sparked one of politics' more unusual conspiracy theories - that he had died and been replaced by a lookalike from Sudan.In recent months, some critics have focused less on his physical frailties and more on his mental faculties.After an appearance in a live televised question-and-answer session in January 2019, one commentator called Buhari "intellectually disabled". Others were less kind.Buhari's opponents say they have been vindicated for claiming before the last election that he was too old to run and had terminal prostate cancer.But his ruling All Progressives Congress (APC) party dismiss the assertions as smears. Dogged by the past? How far Buhari's illness has affected his attempt to end the threat from Boko Haram, tackle corruption, and boost the economy will be a subject of discussion for years. On security, there are worrying signs of a turnaround in initial gains against the jihadists, as the group's Islamic State-backed faction grows in strength in the northeast.New security threats have also emerged elsewhere, including pro-Biafran secessionists in the southeast, and deadly clashes between farmers and herders in central states.Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook. In the north, armed bandits are carrying out kidnappings for ransom and rustling cattle, putting them into conflict with local vigilantes set up because of a lack of police.Buhari, a former army general who led a tough military government in the 1980s, had campaigned on a promise to make the country safer.He also touted himself as a "converted democrat" to persuade those with misgivings that his military past was history.But he has struggled to shake off claims of authoritarianism - particularly in his fight against corruption which critics say has been one-sided against perceived political opponents.His main challenger for the presidency, Atiku Abubakar, has accused him of being "dictatorial" for suspending the country's chief justice just weeks before February elections.Nigerian lawyers said it was an "attempted coup against the judiciary", as the judge could hear any legal challenges to the result.Former president and military ruler Olusegun Obasanjo even accused Buhari of trying to copy the hardline regime of Sani Abacha, by suppressing free speech and silencing opponents. Ruling 'cabal' Buhari came to power largely through the overwhelming support of northern voters, who still see him as a down-to-earth "man of the people".He has done little to challenge that perception, and is rarely seen in anything other than traditional Muslim robes. He has a reputation for a simple lifestyle.But his leadership style has been attacked for concentrating power within a small group of trusted advisors, who like him are Hausa-speaking, ethnic Fulani Muslims from the north.On several occasions, his wife, Aisha, has criticised the so-called "cabal" around her husband, suggesting they held the real power.She has also indicated she would not support him in his bid for re-election, and even retweeted videos of opposition lawmakers criticising the government.Critics, who gave Buhari the nickname "Baba Go Slow" because it took him six months to even appoint a cabinet, complain he has been sluggish to implement policies.Some blamed his economic management after Nigeria's currency crashed and the country plunged into recession.But to some extent the cause of the crash - the global slump in oil prices - was beyond Buhari's control, and forced the government to start thinking about diversifying the economy.Many have been satisfied he at least started to tackle the scourge of corruption, even if at times he appeared to struggle with consensual politics.Buhari, from Daura, in the northern state of Katsina, divorced his first wife, Safinatu, with whom he had five children. Aisha, whom he married in 1989, is his second wife. They also have five children.