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Last Updated: Thursday, 11 December, 2003, 01:26 GMT
Epidemic threat of 'minor' bugs
Petri dish
Scientists do not know which pathogens will strike next
Bacteria and viruses that appear to pose little threat to humans may be on the verge of causing major epidemics, say researchers.

Experts from the US and France have calculated that some bugs are gaining enough of a foothold in humans to pose a significant threat.

Even those thought to be virtually eradicated by vaccines may be only one genetic twist away from returning.

In the journal Nature they predict epidemics are "waiting to happen".

The emergence of the Sars virus took the scientific community completely by surprise, just as the arrival of HIV did decades earlier.

In the case of Sars, it is believed the virus managed to "jump" from animals to humans - then pass from human to human.


There are many "pathogens" - bacteria or viruses that cause disease - which cause only a handful of cases within outbreaks.

Scientists need to find ways to predict which of them is likely to turn suddenly from a minor threat to a public health nightmare.

The researchers in this study, from Emory University in Atlanta, the University of Washington in Seattle and Universite Pierre and Marie Curie in Paris, have worked out a way of calculating the risk from each pathogen.

They can calculate an "emergence" factor by observing when a case of the illness in humans happens, and counting how many other people catch the infection as a result of just this single case.

The higher the number, the more likely it is that the pathogen could be dangerous on a global scale.

This is because, they say, the more people who carry the pathogen, the more opportunities it has to undergo a genetic mutation which will evolve it into an organism far better suited to preying on humans.


Changes in this factor could also happen because of increases in population density or living conditions that makes transmission more likely.

The researchers write: "These long transmission chains provide an opportunity for the pathogen to adapt to human hosts, and thus for the disease to emerge."

They say that diseases such as smallpox, eradicated by vaccination campaigns worldwide, could return in other forms, for example "monkeypox", as immunity in the world population begins to dwindle.

Dr Jim Bull, from the University of Texas at Austin, said the "complacency" of the pre-HIV and bioterror eras was no more.

"It is not beyond imagination that, even with existing technology, methods could be developed for monitoring emerging pathogens.

"The means of identifying these 'epidemics-in-waiting' could become a critical tool in global defence strategy against emerging pathogens."

Call for human virus survey
01 Jul 03  |  Science/Nature
Scientists attack Sars with speed
21 May 03  |  Science/Nature

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