"In the last 20 or 30 years there has been an average of one new disease -- often a fatal and very difficult to treat disease -- emerging every year," he says.
Some of these illnesses have become household words, like HIV/AIDS and severe acute respiratory syndrome, better-known by its acronym SARS -- both of them unknown three or four decades ago.
Other new diseases are more obscure but even more lethal, like Marburg fever.
Population, Climate Trends To Blame
The WHO says there are several reasons why new diseases are emerging so often.
The population boom in developing countries means human beings are settling in many previously wild areas, disturbing natural habitats and clearing a path for new viruses to emerge.
In addition, warmer weather is carrying some tropical diseases northward to more temperate countries.
Also, the improper use of antibiotics means old scourges like tuberculosis are reemerging as public health threats, in strains that are more resistant to treatment.
Globalization Of Disease
But what really concerns scientists at the WHO is the global boom in air travel.
More than 2 billion people per year now travel by air. In the words of the WHO report, it means "an outbreak or epidemic in one part of the world is only a few hours away from becoming an imminent threat somewhere else."
That was illustrated recently by the case of an American lawyer, diagnosed with a highly resistant strain of tuberculosis, who caused an international health alert after he flew on a trans-Atlantic flight.
"That sort of thing, fortunately, is fairly rare. But it makes a good point that there is the potential for even just one individual traveling between countries, carrying an infectious disease, to be a threat to others and to cause an alarm or a potential crisis in another country," Prentice notes.
"If we go back three or four years to the outbreak of SARS in Asia, again the outbreak was largely spread by passengers on a particular aircraft [who were] traveling," he says. "Some other passengers on the same plane were infected. And by the time they got off the plane at their destination airport, that disease had become international."
Prentice says the WHO report is aimed at government health authorities around the world, to urge them to improve their monitoring of infectious diseases and to report outbreaks as soon as they happen.
"The idea that you can control a disease by closing down the borders just won't work anymore," he says. Every country has a global responsibility when it comes to stopping the spread of infectious diseases, as scientists work on cures to eliminate these new threats.
An HIV-positive Ukrainian woman and her daughter (epa)
FACES OF THE EPIDEMIC: HIV-infection rates continue to soar in many parts of RFE/RL's broadcast region, from Ukraine and Russia to Central Asia. RFE/RL frequently reports on the problems associated with the pandemic and efforts to combat them.