Many doctors believed that medical science was on the verge of conquering most infectious diseases in the 20th Century.

But the emergence of new infections, and the resurgence of old enemies mean that the world enters the next 100 years increasingly vulnerable. The problem will be examined in the BBC television programme Horizon.

The twin threats of new viruses such as HIV and Ebola, and the increasing resistance of existing pathogens such as tuberculosis to antibiotics.

And their arrival has taken the medical world completely by surprise.

Of HIV, Dr Tim Curran, an epidemiologist, says: "That something could overcome and overtake other causes of death and come from no-where, from not causing any deaths to being the first cause of death - that wasn't even on the radar screen."

The arrival of killer tropical viruses such as Ebola and Marburg has been blamed on the increasing encroachment of humans into previously uninhabited areas, bringing them into contact with new viruses.

Dr CJ Peters, of the Centes for Disease Control and Prevention in the US, says that even worse viruses could be out there waiting to come into contact with the human race.

He says: "We are setting the stage for the emergence of more and more infections."

Although the origin of the HIV virus is still unclear, some doctors believe that it may have been lying dormant in an unpeopled area of Africa.

And the advent of modern intercontinental travel means that a deadly virus can be transmitted between populations in a matter of hours.

Growing resistance

However, while effective treatments for viruses are still far from perfect, scientists believed that the threat posed by bacteria was a diminishing one.

However the bacteria are showing characteristic resilience as they defeat every measure that scientists can produce.

Evolution has led to strains of "superbug", resistant to virtually every known antibiotic.

Natural selection means that those bacteria who show the most resistance to drugs are the most likely to survive, breed and thrive, while those which cannot cope die out.

So eventually the entire bacteria population gains some degree of resistance.

Professor Robert Daum, from the University of Chicago, says: "Humans also evolve, but the difference is that bacteria evolve every 20 minutes, whereas humans evolve once every 20 years."

The principal bacteria associated with resistance to antibiotics is Staphylococcus aureus, which in some cases has become resistant even to Vancomycin, a powerful and toxic antibiotic seen as a last resort medication.

Strains of resistant bacteria are almost always found in hospitals, where the heavy use of antibiotics makes them far more likely to evolve.

However, there is some evidence that the bugs may be appearing in the community at large, where the effect could be devastating.

Old enemy

However, drug resistance is also allowing the return of a disease which doctors believed had almost been consigned to the history books.

Multi-drug resistant tubercolosis is now a serious public health problem in countries such as Russia and, even in deprived inner-city areas in the US and UK.

In Russia, doctors, finding all antibiotics useless against the illness, have resorted to surgery - cutting out the infected lung tissue - to halt the spread of the disease.

In New York and, to a lesser extent, in London, patients can be forced to complete courses of treatment lasting up to two years to eradicate the bug.

The second part of Horizon, life and death in the 21st Century, will be broadcast on BBC Two at 20 00 GMT on Wednesday 5 January.