Senegal's health minister has confirmed a first case of Ebola, making it the fifth West African country to be affected by the outbreak.
Awa Marie Coll Seck told reporters on Friday that a young man from Guinea had travelled to Senegal despite having been infected with the virus.
The man was immediately placed in quarantine, she added.
The current outbreak, which began in Guinea, has killed more than 1,500 people across the region.
At least 3,000 people have been infected with the virus. The World Health Organization (WHO) has warned it could get much worse and infect more than 20,000 people.
Senegal had previously closed its border with Guinea in an attempt to halt the spread of Ebola, but the frontier is porous.
It had also banned flights and ships from Guinea, Liberia and Sierra Leone - the three worst-hit countries.
The Guinean student sought treatment at a hospital in Senegal's capital Dakar on Tuesday, but did not tell staff he had had contact with Ebola patients in his own country.
On Wednesday, the Guinean health services reported "the disappearance of a person infected with Ebola who reportedly travelled to Senegal," according to Senegal's health minister.
She said the missing person was quickly identified as the Guinean student and he was immediately quarantined.
Concerned residents in Dakar are reported to have reacted to the news with anger.
According to Reuters, one host on a Senegalese radio station asked: "When you are sick, why do you leave your own country to export the disease to another?"
Prof Peter Piot, who co-discovered Ebola in 1976, said the case "is not unexpected" and all countries in the region should be preparing for the worst
"I think it illustrates the ineffectiveness of closing borders and cancelling flights. People will still find a way to get around," Mr Piot told the AP news agency.
Senegal, a major transit hub for aid agencies, has a large Guinean population.
In Guinea, a 24-hour curfew has been imposed in the second city, Nzerekore, because of a riot after the main market was sprayed with disinfectant in an attempt to halt the spread of the virus.
The exact cause of the riot is not clear - some people reportedly feared the spray would spread Ebola, while other chanted: "Ebola is a lie". Police responded by firing tear gas.
The city is the capital of the Forest Region, where the Ebola epidemic has its epicentre - near the town of Gueckedou.
However the BBC's Alhassan Sillah in Guinea says the town has miraculously remained free of Ebola so far.
There have been relatively few cases in Guinea recently, with far higher infection rates in Liberia and Sierra Leone, and six deaths in Nigeria.
On Thursday, the WHO unveiled a plan aimed at stopping transmission of the virus in the next six to nine months.
Among its recommendations, it said countries affected should conduct exit screening to prevent the disease from spreading to a further 10 countries.
The plan calls for $489m (©295m) to be spent over the next nine months and requires 750 international workers and 12,000 national workers across West Africa.
Reproduced under licence from BBC News © 2014 BBC