this summit is about our shared future...,

WSJ |  Ebola outbreaks in several West African nations forced two heads of state to pull out of a historic Africa summit with President Barack Obama, in a reminder of how crises on the continent can undermine efforts to change U.S. perceptions of it.

Ellen Johnson Sirleaf, the Nobel Peace Prize-winning president of Liberia, will stay home to deal with an Ebola epidemic that has killed more than 729 people, her office said. So will Ernest Bai Koroma, president of Sierra Leone.

Their decisions show what this week's U.S.-Africa summit is up against in its aim to reshape how big American companies view Africa. The meeting aims to bring business leaders into contact with leaders of the continent, home to six of the world's 10 fastest-growing economies. Yet weak government, military and public-health institutions leave even some of the most hopeful economies looking vulnerable.

Sierra Leone, a country recovering from war on the back of rich iron deposits, is a prime example. Its economy had been growing at 13.8 % this year. But then Ebola prompted the government to put workers on forced leave and ban public gatherings, moves that will likely curtail growth while showing how fragile Africa's gains remain.

Some in Washington worry that the threats the continent continues to face will overshadow the message that Africa is open for business.

"If that happens—and I hope it doesn't—it will be a tremendous lost opportunity," wrote J. Peter Pham, Africa Director at the Washington-based think tank the Atlantic Council.

Several countries are fighting Islamic insurgencies. There is only one session in the summit devoted to terrorism.

And yet some presidents want to use the summit to talk up the dangers of these insurgencies, in the hope the U.S. will boost aid to their outgunned and undertrained militaries.

President Paul Biya of Cameroon will spend his visit requesting assistance in quelling the Boko Haram Islamist militant group, he said in a speech Saturday. His economy is expanding at a 4.8% clip, the International Monetary Fund says, and several U.S. firms have invested in sizable agricultural projects there.

But the country is also becoming a front for Boko Haram, a sect born in Nigeria that has killed thousands of civilians. Growing numbers of its victims are in northern Cameroon. Last week, the group kidnapped the wife of Cameroon's deputy prime minister, the latest in a string of attacks on political figures.