Many human diseases have their origins in birds. Some of these diseases include malaria, dengue and of course influenza. Some of the diseases use other species as vectors (e.g. malaria and dengue that need mosquitoes) and some have gone one step forward, use aerosols (from sneezing birds and people) to infect other hosts.
The feared bird flu pandemic (it is already a pandemic among birds!) has made us think about the evolutionary significance of disease. Much as we would want to eliminate disease, doctors and public health practitioners will never be able to eradicate disease, for diseases are part and parcel of the evolutionary process.
Diseases tend to select traits in populations and while a proportion of the population will succumb to the infection, the population over time will develop resistance to the pathogen. In short the population will be immune to the disease and the disease organism will live commensally with its host population. A possible example is the common cold. The cold virus almost never kills its host (unless the host is immunocompromised) and keeps on infecting and infecting people.
What causes disease to jump from animals to people? It is obvious that our close contact with animals gives the opportunity. But we are close to animals because we have need for them. We exploit their biodiversity. Our most basic need is for food. Ecological history tells us that the rise of killer infectious diseases was hand in hand with our dependence on agriculture. Tuberculosis came from the animal domestication process as well as rabies, which began to infect humans when they started to keep pet dogs.
Malaria jumped from birds to humans because we cleared the forests for farmland and this gave the malaria parasite its chance to infect humans. Malaria still kills about 4 million people a year.
This bird flu story is a retelling of our earlier history with disease. But we know much better now. Disease and environment health are closely linked and that major changes to the environment as we continued to modify natural habitats, will give other emerging diseases their entrance on the human health stage.