In the marketplaces and at the school gates of downtown Monrovia, Liberians spoke Wednesday of their dread over the return of Ebola after the first case in three months emerged.

Health authorities said on Tuesday a teenager had died after becoming infected by the deadly tropical virus, seven weeks after the west African nation was declared Ebola-free.

Buckets of bleach, a ubiquitous symbol of Ebola's virulence as the epidemic cut a murderous swathe though the country last year, began to reappear outside shops and homes as the capital came to terms with the news that the virus was back.

"No, no, my man. Let's do the Ebola shake because the Devil is back again in our land," said Mamadee Sakor, 43, refusing a friend's handshake.

Monrovians, a naturally tactile people, substituted their elaborate greetings for a sterile elbow nudge as the bodies of their neighbours began to pile up in the streets from August last year.

As the epidemic began to subside at the end of last year the handshakes made a comeback, but the "Ebola shake" was back as Monrovians met in the streets on Wednesday.  

"I am scared -- I am so scared that I don't even know where to start. I am confused," said Jeneba Freeman, 45, a stallholder in the capital's Redlight market.

Liberians were informed on public radio on Tuesday that a 17-year-old boy had become sick with Ebola and died in a village near Roberts International Airport, about an hour's drive southeast of Monrovia.

- Crushing blow -

Health minister Bernice Dahn asked citizens to prepare themselves for further cases, although no new infections had emerged as of Wednesday morning.

The news came as a crushing blow in a country still reeling from an epidemic that wrecked its health service and economy and killed more than 4,800 Liberians before the World Health Organization declared the outbreak over on May 9.

The country had good reason to believe the nightmare was over, with no new diagnoses since March 20.

The government was deploying epidemiologists on Wednesday in an emergency containment operation to trace the teenage victim's final few days and ensure anyone with whom he may have had contact was isolated and under observation.

The country's neighbours Guinea and Sierra Leone are both still battling the outbreak, which has killed more than 11,200 people across west Africa, but the coastal Margibi County where the teenager died is nowhere near either border.

The health ministry said it had no reason to believe he had visited either country and the source of his infection remains a mystery.

Ebola is spread among humans via the bodily fluids of recently deceased victims and people showing symptoms of the tropical fever, which include vomiting, diarrhoea and -- in the worst cases -- massive internal haemorrhaging and external bleeding.

Experts are speculating that the victim could have been infected by an entirely new variation of the virus from an animal such as a fruit bat.

A more worrying possibility is that clusters of Ebola continue to smoulder under the surface, far from the gaze of local or international health authorities.