Madagascar faces a bubonic plague epidemic unless it slows the spread of the disease, experts have warned.
The Red Cross and Pasteur Institute say inmates in the island's rat-infested jails are particularly at risk.
The number of cases rises each October as hot humid weather attracts fleas, which transmit the disease from rats and other animals to humans.
Madagascar had 256 plague cases and 60 deaths last year, the world's highest recorded number.
Bubonic plague, known as the Black Death when it killed an estimated 25 million people in Europe during the Middle Ages, is now rare.
The International Committee of the Red Cross (ICRC) in Geneva and the Pasteur Institute have worked with local health groups in Madagascar since February 2012 on a campaign to improve prison hygiene.
"If the plague gets into prisons there could be a sort of atomic explosion of plague within the town. The prison walls will never prevent the plague from getting out and invading the rest of the town," said the institute's Christophe Rogier.
The ICRC said the 3,000 inmates of Antanimora, the main prison in the heart of the capital Antananarivo, live with a huge rat population which spreads infected fleas through food supplies, bedding and clothing.
The ICRC's Evaristo Oliviera said this could affect not only inmates and staff, but others they come into contact with.
"A prison is not a sealed place, first of all the staff themselves who work in the prison are at risk, and they go home at the end of the day, already perhaps being a vector of the disease," he told the BBC.
"Also the rats themselves, they can go in and out of the jail and also propagate the disease.
"And the prisoners do have visitors who can be also infected, and the prisoners eventually go out as well so we have many many ins and outs for the disease to spread."
The BBC's Imogen Foulkes in Geneva says the eradication project undertaken by the ICRC is tricky because simply killing the rats is not enough.
To prevent their infected fleas transferring to another host, possibly a human, the insects must be destroyed as well as the rodents, she says.
Mr Oliviera said the disease could be treated with antibiotics if detected early, but a lack of facilities and traditional shame over the disease made this tricky in outlying parts of Madagascar.
Experts say that Africa - especially Madagascar and the Democratic Republic of Congo - accounts for more than 90% of cases worldwide.
However in August a 15-year-old herder died in Kyrgyzstan of bubonic plague - the first case in the country in 30 years - officials said
During the last 20 years, at least three countries experienced outbreaks of human plague after dormant periods of about 30-50 years, experts say.
These areas were India in 1994 and 2002, Indonesia in 1997 and Algeria in 2003.
According to the World Health Organization, the last significant outbreak of bubonic plague was in Peru in 2010 when 12 people were found to have been infected.