Zoonoses, diseases that pass from animals to humans, are a growing health problem around the world.

More than 200 known zoonotic diseases have emerged or re-emerged all over the globe.

Between 2000 and 2005, 50 million people caught a zoonosis and 78,000 of them died.

Understanding their sources and effects has become an important issue for global public health.

And what's more concerning is that there are no effective treatments or vaccines for some of the most common zoonotic viruses, like malaria.

Recently, the bird flu virus ( H5N1 ) has attracted attention not only for spreading from birds to humans, but also for its capacity to mutate in such a way that could allows it to spread freely between humans.

In the last years, bird flu killed more than 80 people from Turkey to China and Indonesia.

"This comes on the heels of other major zoonotic viral epidemics in the last decade," said virologist Jonathan Heeney of the Biomedical Primate Research Center in The Netherlands.

These viroses were provoked by SARS (severe acute respiratory syndrome) coronavirus, West-Nile virus, Ebola virus and monkeypox virus.

Malaria still takes a toll of 700,000 to 2.7 million victims (75 % African children) yearly.

But malaria is not considered a zoonosis, as the parasite needs a human host in its life cycle.

Dengue virus (photo), linked to jungle monkeys and transmitted to humans by mosquitoes, has seen a resurgence in the last years.

The cycle often moves into urban areas, where the mosquitoes infect only humans.

In fact, Dengue hemorrhagic fever is the second disease spread by mosquitoes, after malaria, with several hundred thousands of new cases yearly and 40 million cases around the world.

The rise of zoonosis is due to growth around large cities, increased population mix by immigration and failing public control measures.

But also pet trade could trigger the expansion of zoonosis.

In 2003, 50 people in USA got monkeypox from six African pet rodent species.

When animal viruses can develop the needed mutation for efficient transmission between humans, zoonotic illnesses reach epidemic proportions.

Some human viroses, like smallpox, have been greatly eradicated, but related animal viruses, such as monkeypox, when hit people, find them defenseless.

But zoonoses can be even spread by human from some animals to others, being extremely severe on new species.

A deadly parasite called Toxoplasma gondii, taken by human from cats, provoked lethal brain damage in California sea otters.

Also, toxic chemicals and human herpes virus cause cancer in the same species.

"They (doctors) are in the best position to identify trends and patterns, such as increases in the number of deaths of wild or domestic animals," said Heeney.

"Awareness and surveillance of ecosystems will play a key role in identifying and controlling new, emerging and re-emerging viral zoonotics."

"Something new will happen and it will be unexpected," Robin Weiss, Chairman of the Windeyer Institute of Medical Sciences at University College London, UK, and Director of its Wohl Viron Centre.

"Who could have predicted something like SARS?" "It is not the question of 'if', but 'when' the next zoonotic disease will strike."