He told a conference in Canberra, Australia: "One third of people dying today are dying of infectious diseases. This is inexcusable in a world where there are drugs to treat disease and vaccines which can prevent it." Only last month the British Government's Chief Medical Officer, Dr Jeremy Metters, warned that age-old diseases such as diphtheria and TB were returning in Eastern Europe and parts of the Third World.

But Dr Heymann, who took part in investigating the first outbreak of Ebola more than 20 years ago, said the biggest challenge was posed by the evolution of new viruses. "If a new disease emerges, it would take at least six months to develop a vaccine," he added. "We face emerging diseases which don't kill very many people but cause chaos in the world. They close airports and they close countries."

Dr Heymann said an example was the outbreak over the last two years in the Congo of monkeypox, a virus that jumps from animals to humans, which has struck more than 300 people. There is no known cure and it kills at least 10 per cent of people who contract it. There is a vaccine that prevents monkeypox - the same as that used against smallpox, which is now eradicated - but it can kill people with HIV.

Dr Heymann said: "Twenty years ago, we could have vaccinated the entire population for smallpox and prevented monkeypox as well. But we can't do that now because there is HIV in this population." The evolution was intriguing, said Dr Heymann. "One disease HIV/AIDS, appears and that has completely changed how we relate to the third disease, monkeypox. Education of people is now the only way to prevent it."

Even deadlier than cancer

More people in the world die from infectious illnesses than either heart disease or cancer, Western Europe's two big killers. Last year 17 million people succumbed to infectious and parasitic diseases, many of which are easily preventable with vaccines. The top 10 killer infections are:

Thanks to the World Health Organisation
for much of this information.