This has been one of the great killers in human history with 20-30 million deaths after the First World War alone.

New epidemics of typhus are now raging in Burundi in Central Africa, and threaten to surface elsewhere in Africa and among homeless people in Southern Europe.

It is spread by lice and, left untreated, can lead to death within three weeks.

Symptoms will include fever, delirium, and rash. The brain, heart, lungs, and kidneys can also be affected.

The small bacterium responsible is called Rickettsia pronazekii, and the complete sequence of all its 834 genes will now help scientists develop better treatments and, in particular, an effective vaccine.

DNA techniques

"It becomes much more straightforward now to generate vaccines, either by traditional means or the new DNA techniques," said Dr Charles Kurland of the University of Uppsala in Sweden, who led the research effort.

"It becomes possible to study targets for new antibiotics, it becomes possible to study the epidemiology [distribution and spread] of the disease much more easily now we have genes to use as markers, and it should certainly help us to prepare ourselves for any re-emergence of the disease."

Typhus is thought of as one of "yesterday's diseases" in the West. But in those parts of the world afflicted by war and famine, epidemics of typhus are an ever-present threat.

However, there are now fears that typhus may also surface in parts of Europe where large numbers of homeless people are sleeping rough in the streets, making them vulnerable to infection with lice.

Vaccination programmes

Antibiotics cannot control large-scale epidemics because people who are cured are constantly re-infected. Vaccination, which hopefully protects for life, is the best weapon against epidemics, especially in poor and remote areas.

"If we look at the same countries, Rwanda and Burundi, there have been several outbreaks of disease - cholera and then meningitis - and the meningitis was well controlled by vaccination," said Professor Didier Raoult of the World Health Organisation's Reference Centre for Rickettsial Diseases.

"You go from place to place vaccinating and when you have vaccinated everybody the problem is definitely solved. You don't have to come back and come back and come back.

"When you use drugs to treat typhus, or insecticide [to kill the lice that spread the disease] the problem is that six weeks later you have to come back and do the same thing over and over and over."

Experts now hope pharmaceutical companies will be encouraged to put far more effort and resources into developing new medicines.

Rickettsia pronazekii is named after H.T.Ricketts and S.J.M.Prowazek, both of whom died of typhus while studying rickettsial diseases early this century.

Charles Nicolle was awarded the Nobel Prize in 1928 for discovering that epidemic typhus is transmitted by lice.