New infectious diseases are now emerging at an exceptional rate, scientists have told a leading conference in St Louis, US.
Humans are accumulating new pathogens at a rate of one per year, they said.
This meant that agencies and governments would have to work harder than ever before to keep on top of the threat, one expert told the BBC.
Most of these new infectious diseases, such as avian influenza and HIV/Aids, are coming from other animals.
"This accumulation of new pathogens has been going on for millennia - this is how we acquired TB, malaria, smallpox," said Professor Mark Woolhouse, an epidemiologist at the University Of Edinburgh, UK.
"But at the moment, this accumulation does seem to be happening very fast.
"So it seems there is something special about modern times - these are good times for pathogens to be invading the human population."
Professor Woolhouse has catalogued more than 1,400 different agents of disease in humans; and every year, scientists are discovering one or two new ones.
Better surveillance:
Some may have been around for a long time and have only just come to light.
Others that have emerged recently are entirely new, such as HIV; the virus that causes Sars, and the agent of vCJD. The difference today, say researchers, is the way humans are interacting with animals in their environment.
Changes in land use through, for example, deforestation can bring humans into contact with new pathogens; and, likewise, agricultural changes, such as the use of exotic livestock.
Other important drivers include global travel, global trade and hospitalisation.
The fast rate at which pathogens are appearing means public health experts will need to work harder than ever to control the spread of emerging disease threats.
"The sort of image I want to get away from is the famous statement from the 1960s when the US Surgeon General said, 'diseases were beat'," Professor Woolhouse told the BBC News website.
"Pathogens are evolving ways to combat our control methods. The picture is changing and looks as if it will continue to. We're going to have to run as fast as we can to stay in the same place."
He added: "We need surveillance. Surveillance in most parts of the world for infectious disease is really quite poor - particularly surveillance for infectious diseases in animals such as vermin like rats."
Experts were speaking on the subject at the American Association for the Advancement of Science (AAAS) annual meeting in the Missouri city of St Louis
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