Health officials in Canada are very busy these days. They are placing chickens at fixed points all along their border with the United States of America. That's an enormous distance of 2,500 km.

It's not a practical joke, nor have the Canadians gone mad. They are using these chickens to see if the deadly West Nile virus is lurking around. The virus infects birds, so they think that the chickens have a good chance of catching the virus. Or the virus will catch the chickens.

The Canadians are worried because the West Nile virus is transmitted to humans by mosquitoes. It killed seven people in New York last year.

Countries around the world are realising something important. That it may just be possible to stop certain kinds of people from entering their land, but it is very difficult to try and stop viruses that travel from one end of the earth to another. When they travel to new places, they adjust very easily to those climates and sometimes start destroying the local plant and animal life. These biological polluters are called smart polluters.

These smart polluters can be carried across borders of countries unknowingly. Just as we humans are travelling across the globe more often than earlier, these biological polluters have also started journeying much more. They travel in the ballast of tankers. They slither into aircraft through their wheels. They bore their way into objects that air travellers may be carrying from one country to another.