Current measures against Avian Influenza severely threaten domestic poultry diversity-the photo shows a Pomeranian Duck，a breed threatened by extinction. Photo：Feldmann/GEH

Key terms

Immunity is a human’s or animal’s insusceptibility to a certain disease. It can be congenital or acquired through vaccination.

Resistance is the ability of a host to resist infection or control the lifecycle of a parasite.

Tolerance means that an infected animal displays few measurable symptoms of a particular disease.

An infection or disease is endemic if it occurs in a particular location or population with predictable regularity.

A disease is called an epidemic if it infects many animals at the same time and spreads rapidly.

An epidemic becomes a pandemic if it spreads all over the world or over several continents.

Measures designed specifically to prevent the spread of an infectious agent are termed biosecurity measures.

Zoonoses are animal diseases which can also be transmitted to humans；they include rabies and anthrax.

In vitro conservation describes the preservation of genetic material，for example in a seed bank.

In vivo conservation，in the context of the conservation of animal genetic resources，describes the keeping and breeding of animals，for example on a farm.

Cryoconservation is the deep-freezing of sperm or embryos.

Avian Influenza is the latest animal disease that has frightened many people as we watched it spread across Asia and Europe. Apparently，not even the pre-emptive culling of chickens，geese，ducks and turkeys-whether infected or not-can halt its spread，even though that is the exact objective of culling. In the past decades the carousel of animal diseases has spun ever faster. According to the United Nations Food and Agriculture Organization（FAO），every year on average one new animal disease has emerged. Three quarters of these were zoonoses.