Few things on Earth are spookier than viruses. Not surprisingly the word virus means "poisonous slime" in Latin.

They are very small – billions fit in one drop of water.

They have no means of locomotion, yet they travel around the world, by hitching a ride with living things.

Stripped down to the bare essentials, they have no need for movement, breathing, growth – their only function is to create other viruses like themselves.

Not quite alive, but not really dead, these strange entities have caused unimaginable suffering, disfigurement and death throughout history. And today, the world watches and waits for another pandemic to sweep the planet.

But for all their destructive power, many scientists believe that viruses are the earliest form of life - our ancestors - responsible for our very lives, and that of all life on Earth. In fact, viruses are the number one entity on the planet.

Ken Stedman is a particular breed of virus hunter. He hunts extreme viruses that live in the boiling acid pools of Lassen Volcanic National Park in California – viruses that have been around since the birth of the planet. What these viruses tell us about life on Earth is that they are here to stay. So we better be prepared to deal with them.

Donald Henderson knows that all too well. This passionate virus hunter is responsible for one of the greatest accomplishments in human history. Henderson is the man who led the global eradication program against smallpox.

His is a fascinating story of dedication, diplomacy and sheer grit. But despite Henderson's breathtaking victory over a disease that killed 300 million people in the 20th century alone, smallpox remains the only virus that mankind has conquered.

Within a year of the eradication of smallpox, triumph turned to terror, as AIDS emerged from the shadows to become a worldwide pandemic. Ebola, Marburg, SARS, West Nile Virus, and a host of new infectious diseases soon followed. There have been 30 new infectious diseases that have shown up in the world since smallpox was wiped out, as well as at least 20 re-emergences of known diseases.

Steven Jones is a highly specialized Canadian scientist – an infectious disease firefighter, sent to Angola to try and contain the deadly outbreak of Marburg virus in 2005. The virus is the stuff of nightmares. Jones is part of the WHO's global response team, set up to deal with such horrors.

We follow Jones in this life or death battle with the lethal virus, his triumphs and defeats, the challenges he and his team face in developing vaccines, and the limitations of this approach.

Kristine Smith is part of a new generation of virus hunters. She is a veterinarian from Wildlife Conservation Society in New York, and is traveling to the ends of the Earth to the front line in the battle against avian influenza. We follow Smith to the wilds of Mongolia, where she is part of an interdisciplinary and multinational team.

Veterinarians and wildlife biologists have become the new first line of defense against the infectious outbreaks that plague mankind. Close to 75% of all the new human diseases are due to diseases transmitted by animals. But Smith and the members of her team are also conservationists. In their battle with avian influenza, they are forcing a growing understanding that man is bringing many of these diseases on himself –through legal and illegal wildlife trade, through travel, urbanization, population growth and environmental destruction.

And finally, the documentary explores the silent epidemic that is hitting us much closer to home, every day - the age-old threat of chronic diseases – cancer, stroke, heart disease, mental illness - more and more, these diseases are now suspected of being caused by viruses. Almost 20% of cancers are now known to be caused by viruses. A new front has opened up in our intense arms race with microbes. There are investigations going on into all kinds of diseases, from diabetes and multiple sclerosis to schizophrenia and autism.

Jim Laidler is a virus hunter in training. He is the father of two autistic boys. It seems increasingly likely that autism is caused by infection during pregnancy. But this medical doctor quit his lucrative job, and is now traipsing around Lassen Volcanic National Park as Ken Stedman's graduate student. After years of frustration, he has decided the only way to find the truth about autism is to do his own research.