According to scientists in the U.S. and Scotland, 75% of the 38 species of harmful organisms and viruses identified in the past 25 years are believed to have "jumped" from animals to humans.
The researchers say that every year at least one new disease is jumping the species barrier from animals to human beings, exposing people to new germs at a rate that may be unprecedented.
They say that should the potentially lethal H5N1 strain of bird flu mutate into a virus that can spread between humans, it would simply be part of an alarming ongoing trend.
Scientists now know of more than 1,400 kinds of bacteria, viruses, protozoa, fungi and worms that can affect human health.
In a survey, led by Professor Mark Woolhouse, of the University of Edinburgh, more than 1,400 pathogens that can cause disease in human beings have been identified and at least 800 of those have crossed the species barrier from animals.
Professor Woolhouse says the rising number of infections passing from animals to humans could be due to changes in human behaviour and the environment, such as hunting, intensive agriculture, the ease of long-distance travel and global warming.
The expansion of cities into natural habits such as rainforests also has an impact by helping animal germs to acquire the ability to infect humans.
At the annual meeting in St Louis of the American Association for the Advancement of Science (AAAS), Woolhouse said that the apparent rise in the number of pathogens, agents such as bacteria that transmit disease, was "too fast" simply to be caused by natural processes such as evolution.
His research, is the first to catalogue the range of germs capable of infecting people and has revealed that a vast range of animals carry organisms that could infect humans.
Animals apparently spread a range of diseases contacted from cows to chickens, rats and bats.
Diseases carried by animals include the SARS virus, tuberculosis, smallpox, measles and AIDS.
The deadliest example of a germ that has recently crossed from animals to human beings is HIV, which is thought to have started out as a monkey or ape virus in Africa.
Other conditions with an animal origin include the Ebola and Marburg haemorrhagic fevers, the coronavirus that causes SARS, the West Nile virus that is now endemic in the United States and H5N1 avian flu.
The RNA viruses are the ones most responsible for new human infections such as HIV and influenza.
They appear to be very adept at jumping the species barrier because they have small genomes that mutate easily, allowing them to adapt to new hosts.
Woolhouse believes the potential of animals to create new diseases means doctors and vets need to work more closely together to detect and fight such pathogens.
Nina Marano, of the U.S. Centres for Disease Control and Prevention, Atlanta, agrees and says public health and veterinary medicine need to be brought closer together.
The research is published in the journal Trends in Ecology and Evolution.