The world is facing a growing threat from new diseases that are jumping the human-animal species barrier as a result of environmental disruption, global warming and the progressive urbanisation of the planet, scientists have warned.
At least 45 diseases that have passed from animals to humans have been reported to UN agencies in the last two decades, with the number expected to escalate in the coming years.
Dramatic changes to the environment are triggering major alterations to human disease patterns on a scale last seen during the industrial revolution.
Montira Pongsiri, an environmental health scientist at the US Environmental Protection Agency in Washington, says that previous transitions in human history have had a devastating impact in terms of the spread of disease.
"We appear to be undergoing a distinct change in global disease ecology. The recent emergence of infectious diseases appears to be driven by globalisation and ecological disruption," Dr Pongsiri said.
He and eight colleagues examined five emerging and re-emerging diseases -- malaria, lyme disease (spread by ticks); Hantavirus (spread by mice and rats); West Nile disease (spread by mosquitoes); and schistosomiasis (spread by freshwater snails). They argue that changes in land use, farming practices and climate lie behind the increasing number of outbreaks.
The best known example of a disease that jumped the animal-human barrier and went on to cause a global pandemic is HIV, the virus that causes Aids. HIV is thought to have crossed from chimpanzees to humans in West Africa in the last century and more than 25 million people worldwide have since died from it.
The swine flu pandemic that emerged in Mexico last March also resulted from the mixing of viruses that infected pigs, birds and humans to create a new pandemic strain.
Although it turned out to be milder than expected, future flu pandemics are expected in the coming decades that could have higher death rates and infect millions more people.
Dr Pongsiri and colleagues say that the number of people who succumbed to infectious diseases plummeted in the developed world during the industrial revolution, but the rise of manufacturing and pollution levels increased the incidence of chronic diseases including cancer, allergies and birth defects.
Now, we are in the grip of another epidemiological transition driven by the destruction of plant and animal habitats, the loss of species and changes that have brought more humans into closer contact with animals than at any stage in human history, they say in the journal Bioscience.
David Murrell, University College London lecturer in ecology, said: "Since 1940, over 300 new diseases have been identified, 60pc of which crossed to humans from animals and 70pc of these came from contact with wildlife. I would expect this to continue in this century."
A key factor has been increasing urbanisation, which has resulted in humans moving into previously untouched areas where they have come into closer contact with animals.
At the same time, globalisation has meant newly emerged diseases have transmitted faster and more widely than in the past. "Before the world became so interconnected, deadly and newly emerged diseases were not capable of spreading widely," Dr Murrell said.
"Now it is very possible that they will spread across countries and continents within days, thereby sustaining the outbreak.
"We don't know what's out there or how it might transmit. It is very difficult to predict. The problem is if we deal with a threat successfully it leads to complacency. But these things are potentially serious."
Jan Slingenbergh, of the UN Food and Agriculture Organisation, said changes in animal farming have altered the circulation of viruses and bacteria including influenza capable of infecting humans. In countries like China, higher demand for poultry meat has led to a massive rise in populations of domestic wildfowl.
In southern China alone, there are now around 700 million domestic ducks. In the case of flu, a growing number of viral subtypes have moved from wild animals into farmed and domestic waterfowl, which live in closer contact with humans.
The behaviour of these new viruses is unpredictable -- scientists don't know how likely they are to jump the species barrier into humans. But with more of them circulating, there is a higher chance of this happening, Dr Slingenbergh said. Flu viruses are getting closer to people and food, and agriculture practices are to blame.
He said: "There is no evidence to suggest this is going to end any time soon. Agriculture looks set to continue growing for another two decades, and we are only at the beginning of climate change."
The emergence of HIV, the virus that causes Aids, is blamed on human incursion into the forests of West Africa, driven by pressure of population. Rising demand for food led to the growth of the bushmeat trade, and the slaughter of chimpanzees thought to be the animal reservoir for the HIV virus.
People who ate chimpanzee meat, or whose blood got into cuts or wounds, were exposed to novel infectious agents providing the ideal breeding ground for the development of the new disease.
Aids is thought to have emerged during the latter half of the past century, but was not identified in humans until young gay men began dying of a mysterious illness in San Francisco in the early 1980s. More than 25 million people have since succumbed to the disease.
In May 1993, a young, fit Native American in the south-west of the US developed a flu-like illness. He was rushed to hospital in New Mexico, but died very quickly. It was the first recognised case of Hantavirus pulmonary syndrome, a disease marked by high fever, chills and breathing problems, spread by mice and rats. In addition to the pulmonary illness, which kills four out of five people infected, hantaviruses also cause haemorrhagic fever, resulting in widespread internal bleeding. There is no treatment.
Bird flu could become the 21st-century plague. The virus, which has devastated flocks of chickens and ducks in the Far East, poses the greatest potential threat to the human race. Since 2004 it has infected 442 people and claimed 262 lives, a 60 per cent death rate.
Rabies is the most lethal disease known with a near 100pc fatality rate. It is associated with dogs, but has begun infecting other animals in recent decades thereby increasing the threat to humans.
Malaria is transmitted by mosquitoes and is confined largely to the tropical and sub-tropical areas of sub-Saharan Africa, India, Bangladesh, South-east Asia and central America. At least 14 people have caught malaria in Britain in the past 30 years as a result of being bitten by mosquitoes that have been brought in on aircraft.
Malaria causes at least one million deaths a year.
West Nile virus
The disease, which is transmitted by mosquitoes, caused panic when it appeared in New York in 1999 and 2000, apparently carried by ships bringing exotic birds into the country for collectors. Central Park was closed and aggressive spraying was carried out to kill the mosquitoes.
More than 50 people were hospitalised in New York, and at least 10 died. In 2002, more than 3,700 cases were recorded across the US.