Radiation for example, is invisible, has no smell or odor, while it is extremely deadly for the humans.

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H5 virus which is responsible for avian flu, a disease that has forced a mass slaughter of poultry in many countries and could, if it mutates, cause a deadly epidemic among humans.

ECONOMIST: The World Health Organisation

Most of the viruses are harmless, but some are potentially deadly when passed to humans.

CNN: Tracking deadly viruses' spread from animals to humans

Scientists speculated 10 days ago that the bovine disease is linked to the deadly Creutzfeld- Jakob Disease in humans, sparking the panic.

CNN: France supports slaughter of British cattle if necessary

Fears remain high in a country where deadly viruses have jumped from animals to humans before.

NPR: China's Bird Flu Response Shows New Openness

Unlike cold viruses, which have been tested in humans, influenza is a potentially deadly virus.

FORBES: Coughing Mannequins Shed Light On Flu Transmission

As well as logging and farming, humans are also introducing new species that are proving deadly to big trees.

BBC: Why are the world's older trees disappearing?

France hopes to test 20, 000 animals every week for the deadly, brain-wasting illness, which scientists believe can be passed on to humans via infected beef.

CNN: Beef sales slump after EU BSE scare