Brewing in the far corners of the globe, in hot spots in Southeast Asia and Africa, are viruses whose feared impacts are large enough to place them in the crosshairs of health experts.

Many of these are emerging zoonotic viruses -- a virus transmitted to humans from animals.

For its exposure to the wildlife trade, from which several recent diseases such as SARS have emerged, Wildlife Trust, a New York conservancy organization, has been developing a global early warning system to trace brewing diseases.

An example of one it's tracking is Nipah virus in Bangladesh, says Peter Daszak, president of the Wildlife Trust.

Fruit bats are the natural hosts for Nipah virus. Considered a hemorrhagic fever, it causes severe illness in humans, characterized by inflammation of the brain (encephalitis) or respiratory diseases.

The first outbreak was in Malaysia in 1999, however, the majority of the dozen outbreaks since then have been in Bangladesh. There have only been about 500 cases, but the case fatality rate is estimated at 40% to 75%, according to the World Health Organization (WHO).

Elsewhere, on the African continent, Ebola virus is a similar hemorrhagic fever. Infections have been linked to the handling of infected chimpanzees, gorillas, and forest antelopes in Cote d’Ivoire, the Republic of Congo, and Gabon. Like Nipah virus, the Ebola host is thought to be fruit bats.

Ebola was first identified in 1976 and the last outbreak was in 2003. About 1,850 cases with more than 1,200 deaths have been documented since it was discovered, according to the WHO. Like Nipah, occurrences have been rare, but their deadliness warrants detection.

Other examples of rare hemorrhagic fevers are Crimean-Congo, Marburg, and Rift Valley. (More widespread fevers are Lassa and Dengue, which afflict millions yearly).

The economic impact of these rare viruses has naturally been muted so far. They are too lethal for their own good, say health experts, so they don't spread rapidly between humans and create large outbreaks.

"They’re not very important from that standpoint,” says Peter Hotez, professor and tropical-disease expert at George Washington University. “They have minimal impact on economic development. They tend to cause small outbreaks -- a few dozens or hundreds of people -- and then go away. Emerging infections are not really as significant” as malaria or other common tropical diseases.

However, that isn't preventing people from tracking them. The WHO considers them of public health concern because they are so deadly. And Daszak says the financial impact of an outbreak in developed countries -- and the resulting fear -- could be devastating.